In a word: NO.
If you have any other grounds for suspecting her of cheating then you need to explore those, but you absolutely MUST dismiss any thoughts that your gf is cheating based only upon the idea that you think she feels looser.
As the boys who know are telling you, a girl's vagina will vary in size according to her state of arousal. So you might be better off congratulating yourself for turning her on rather than getting paranoid and suspicious - which is bound to turn her off.
Also, bear in mind that if you're doing pe your cock will sometimes be fuller than usual, and at other times it will be exhausted and not achieve full erection. Do these differences in your performance cause her to suspect you of cheating? Of course not.