Boyfriend making sexual advances repulses me

I'm basically just wondering if this has happened to anyone else throughout their pregnancy. I'm almost 33 weeks and everytime my boyfriend does something to initiate sex it makes me really uncomfortable. Like he will casually reach down my pants or my shirt and try to rub my nipples and that use to turn me on but now him touching me like that makes my skin crawl. Is this just a pregnancy thing? I don't like feeling this way about him but I don't want to let him do something while I'm sitting there hating it