Well, other than the roommate and the MIL you just described my experience with garlic!
I identified garlic as a major migraine trigger for me about 10 years ago. At the time those nasty roasted whole heads of garlic were all the rage as appetizers, and there were times I couldn't even be in a restaurant without the smell triggering me. The good news is that as I've been conscientious about avoiding garlic it's effect has gotten much milder when I accidentally encounter it.
I don't know if triggers run in groups, but you might want to watch to see how onions and bell peppers effect you - for me they are triggers, especially when cooked - I've figured that the cooking process releases some sort of oil that is what is my actual trigger. I can eat raw bell pepper most the time, but cooked ones put me out of business for a day.
Oh, and you can tell your MIL that you have a food sensitivity to garlic, if she doesn't understand migraine triggers that is. And if she persists, I'd point out that intentionally dosing someone with a substance that they are known to be intolerant or allergic to is tantamount to poisoning.
Good luck! I hope that now that you have identified what seems to be a MAJOR trigger you will have fewer migraines and be able to experience a more normal life!
Oh, and just one more totally random thought - migraines are a type of inflamatory reaction (I think) - all the current buzz about research showing that inflamation is a major contributor to obescity has got me wondering about if my migraines used to be one of the reasons I struggled so much with my weight. Just food for thought!