If it's causing you this much anxiety, perhaps you should suggest to your boyfriend that you don't want to go. Valentine's Day should be for you and your boyfriend; not his mother If she knows you're on a LC plan, then you'd think she would have been courteous enough to ensure that there was an alternative for you (same if you were a vegetarian right?). I don't think I would like Valentine's Day much either if I were pressured to go out and eat big meals with family... A romantic night with my girlfriend at home sounds much better to me.

Good Luck
-Drew