Tinder

Kourtney
So I've been dating my boyfriend for well over 2 years. And my "best friend" texted me saying that her "friend" saw my boyfriend on tinder and that he has been messaging girls. 
She has 0 proof. Won't tell me any info. And expects me to believe her...
My boyfriend lets me use his phone, I know his Facebook and email passwords. We live together. 
When I asked him he answered all my questions. Never rushed me or tried to get me to stop. 
My gut says he didn't do anything wrong. But fuck did that thought hurt me .. 
Am I being nieve? 
Additional info: he told me that he had tinder between breaking up with his ex and meeting me.  I don't know how tinder works but he said he never deleted the account. Just deleted the app. So he said it may show he is back in the valley where we just moved last month !