Unless there was sugar in your cream, there's nothing particularly sweet about this combination. Did you salt the pasta water to roughly seawater saltiness? If not, did you adjust salt before serving? If your pasta water was under-salted, that would explain the sweetness, since the only significant source of salt otherwise was the parmesan.
Heavy cream is equivalent to 36-40% whipping cream.
In the event that your cream was pre-sweetened, there's not much you could do except add salt and hope the sugar doesn't overwhelm. It's a fairly common technique to add sugar to a salty dish to make it taste richer, or salt to a sweet dish for a similar purpose. But it wouldn't work very well if the cream was very sweet. Since I've never seen sweetened whipped cream except in aerosol dispensers, I can't say for sure.
Sunflower lecithin (sorry, best link I could find) is gaining in popularity as an alternative to soy lecithin because it is widely perceived to have a neutral taste and actually has superior emulsifying properties. It's a little on the expensive side, though.
Soy lecithin by itself doesn't taste horrible if you buy it as a food additive (as opposed to a nutritional supplement). It's common to find in a lot of popular frozen pasta-and-sauce dishes, such as Michelina's, which obviously must be microwaveable without the sauce curdling or separating.
Another option is Sodium Stearoyl Lactylate, if you can get hold of it. This one adds a little sweetness which is generally pleasant-tasting, or at least inoffensive. You'll find it used most often as a dough conditioner or in baked goods, but another popular use for it is in commercial sauces, although unfortunately I can't find specific examples right now.
But probably the best emulsifier for what you're trying to do here is xanthan gum. It's also used as a gelling agent, but in small quantities works as just an emulsifier. You can find it, for example, in this Bertolli Creamy Alfredo Pasta Sauce - exactly the sort of thing you're making. It's also used in the Stouffer's Alfredo sauce.
Mix in a little lecithin or xanthan gum as an emulsifier while the sauce is fresh or cooking and it's very unlikely that your sauce will separate, either in the fridge or in the microwave.
As for what you can do to specifically prevent separation while reheating, if you didn't add any emulsifiers in the first place... I really don't think there's anything you can do. If you're a slow enough eater the sauce will eventually separate right in front of you; that's just what happens when you have water and fat in the same sauce. Best to reheat on the stove top in that case, and stir frequently to prevent any further separation.
Best Answer
According to Cooking The Roman Way by David Downie, Fettucine Alfredo is a traditional Roman recipe called "pasta del cornuti" (cuckold's pasta). What either Alfredo Di Lelio III, or Mario Mozzetti, depending on whom you believe, invented in 1914, was the dramatic tableside preparation of Fettuccine Alfredo, which is what made the dish a hit with visiting Americans in the 20's and 30's.
The tableside preparation is really what makes Fettuccine Alfredo what it is: the hot pasta is tossed with the butter and cheese in front of the diner, and then served to them immediately.
Again, according to Downie, the only ingredients of Fettuccine Alfredo are egg fettuccine, lots of butter, lots of Parmegiano-Reggiano, and (if necessary) a little salt.
Recipes which add cream or milk are Americanized recipes designed to allow restaurants to hold orders of Alfredo for a long time under heat lamps (and turn it into a gooey pasty mess). Italians, from my experience visiting Italy, rarely put cream or milk on pasta (a real Italian could speak up here).
Downie has a fun 3-page digression about the ongoing lawsuits between the Roman families who claim to own the name. It's worth a read.
Alan Davidson, predictably, says nothing about Fettuccine Alfredo. The Glorious Pasta of Italy likewise does not cover the dish.
Wikipedia supports Downie's story, except only attributing Di Lelio, and adding the tidbit that Di Lelio apparently called it "Fettuccine al burro" (fettuccine with butter), and the Alfredo name was appended later when it was copied in the USA. Wikipedia also says butter and cheese only, on fettuccine pasta.
So, to answer your question and the questions asked in the comments: