Heh, you remind me of myself when things went sour with Jack. I was paranoid clear up until the very end, mainly because like your gf, he was acting very strange.
There's no real way to tell whether or not she's losing interest, maybe seeing someone else, or she may just be stressed out over you not trusting her. There could be LOTS of reasons why people distance themselves. Some people get distant when their hearts are broken. I don't know. In my opnion, if you two break up, you're going to end up trusting her a lot more than you did when you two were dating. As far as being suspicious, I don't blame you since she's acting strange, but like I said, she may be acting strange for a completely different reason than you expect. Who knows, maybe she enjoys making you jealous.