Husband lying ....

My husband said he doesn't use what's app but he fell asleep and I looked at the time on his phone and there is a message from a girl saying with more kisses than i care to count. I can't remember the message exactly but it was along the lines of, work hard today?x  Hope you have a good night xx goodnight xxx
He says its a mistake because he doesn't use it but he's obviously had a conversation with her. I'm beyond livid, I couldn't even get on to check the rest of the messages because he has a password on his phone that he won't tell me. 
We've been married nearly 2 years and I'm pregnant with my second child. Is it just the hormones or would anyone else think this is out of line? He told me he doesn't use what's app at all