Rivka,
I can relate to you very much, as I, too, have gained a lot of weight (25 lbs) recently due to IR and PCOS, so I look "different" now than I did when I was thinner (and prettier). Like you, I now feel very uncomfortable in social situations, especially where there are people who used to know me when I was thinner or people who are especially judgmental about weight and appearance.
Just to give you an example: about 3 months ago, my dbf's daughter was getting married, so of course, he and I were invited to the wedding. I was stressed about going because I have gained so much weight and his two adult daughters (both gorgeous and thin) and ex-wife (who always looks great) was going to see me and wonder why I have gotten so fat since they last saw me (and also wonder why their dad and ex was still with me). And there was bound to be a LOT of photos taken and I didn't want to be the fattest, bloated-looking one in all of them!
Well, since there was no way I could get out of going to the wedding, I decided to do what dillydaydreams mentioned and look as nice as I possibly could. I bought a fabulous designer dress that was flattering on me, cute shoes, and had my hair and nails professionally done. Looking as good as I possibly could gave me a little added boost of confidence; I was still a little nervous at the wedding but I concentrated on smiling a lot and making small talk with the friendliest people there I could find. My dbf knew I was anxious, so he stuck by me as much as he could (being the father of the bride and all) and either had his arm around me or held my hand frequently.
As it turned out, his daughters and ex-wife were so busy and stressed with the wedding, I don't think they noticed me much. And there was a very nice Greek lady at the wedding who kept complementing me on my dress and hair despite my obvious weight gain. So, I made my dbf happy by being at the wedding with him and I was able to survive the whole thing with my dignity and self-esteem still intact.
Sorry this story was so long, but I just wanted to make the point that we all have to deal with those uncomfortable social situations from time to time. And despite our PCOS and other related illnesses changing our appearance, we still have to live our lives and inevitably face the "thinner and luckier than us" people in our lives.
In your case, my guess is that your family would love for you to be there and although they may notice your weight gain, they'll probably notice more whether or not you look happy. So go to your Thanksgiving dinner with a big smile and be proud of yourself regardless of what others think! If people there are giving you funny looks, then they are small in character and don't deserve having a wonderful person like you in their lives anyway!