FURIOUS AT SO! Should I be?

Kristina • Mama to 2 boys & 3 step kids
I'm posting this because I'm not sure if my pregnancy hormones have taken over or if I'm actually really pissed. 
I woke up today to a Facebook notification of some random girl liking a picture of me and my SO. Neither one of us are friends with her in Facebook, and it was an old picture which means she had to have been snooping though a bunch to find one to click like on. Right?
Well I then asked him if he knew her. He said yeah, hasn't seen or heard from her in 5 years. I still thought it was weird that she would snoop through either mine or his page to like an old picture. I then asked if he used to hook up with her, that'd be the only thing to explain it. If it was a yes then so be it, it's the past, but at least it would offer an explanation as to why she's snooping. He assured me 3 times he never hooked up with her. 
It still bothered me because in my gut something was off. On the way to work I called him and asked one last time if he had ever hooked up with her. He finally confessed yes and that he lied because he didn't want to get into it or have me feel some type of way. 
HE LIED! 3 TIMES! He's apologized and said he was wrong for lying but he just didn't want me to start thinking about some girl he's hooked up with. So apparently he lied for my sake. I'm so mad at him for not just simply telling me the truth from the jump. I wouldn't of even been mad. I just wanted an explanation as to why some random girl might snoop. 
Granted, this is his first lie that I know of. Now my mind is questioning if I can forgive him, trust him or believe anything he says. 
Advice please. Is this something you'd be mad at? Would you let it go? Blame it on my pregnancy hormones?