why does my 22 year old husband not want to have sex with me anymore?

i gained alot of weight when i was pregnant with my son and i am gaining weight again as i am pregnant with # 2. i feel like he thinks i am not good enough anymore and he is not attracted to me. he says he just is not that into sex, but he really used to be. he loves me i know that but i dont think that i should have to beg for sex and then i still dont usually get any.