Is it normal to feel so insecure?

Been with my man six years, we live together and have a child together. Through our entire relationship i have been faithful. Our relationship has had its ups and downs, but I've always been adventurous and kept him happy in the bedroom (its never been a problem) but lately my gut instinct don't feel right, i have a feeling that hes playing around behind my back, he is always late home from work, and when he gets in he jumps straight in the shower, he is never around for me and our child, never makes an effort with me. He answered his phone by accident but i could hear a female in his car (but he said it was just a client) he says im going crazy and its all in my head. Ive also found white stains in his boxers which ain't nothing to do with me. However im not normally an insecure person, but something is telling me this ain't right. My partner is always saying how lucky he is to have me, i get alot of attention and im not big headed but i am a very attractive girl with a slim body. I don't want to leave him but if i find out he has cheated i will never forgive him and would end it straight away and he knows this. I really cant get this gut feeling to go, hes also started to get argumentive with me and snappy. Does this sound like hes cheating? And if he is cheating he will not own up to it, so how can i start gathering evidence? Ive searched through his phone but i have noticed he deletes all his calls and messages and emails, which is weird. Advice please