Lying about Dating History

My intuition told me my boyfriend wasn't fully transparent about one girl he'd dated between his ex and before me - he got nervous and quiet and said "it wasn't deep" - minimising it.

this girl was his sister's best friend

understandably, i worry i will see them when we visit his home town.

he eventually admitted to me last night after asking AGAIN about her that i didn't feel he was telling the truth. Turns out they dated a month after his ex (breakup in december, dating new girl in january) and it ended with her in april (he met me in may)

I feel like he has been a coward consistently lying to me about it. One month between all of us is not a lot. He said he was protecting my feelings - that he lied about it at the beginning because he didn't want me to not date him, i feel like i've been deceived.

He asked her gto be his girlfriend in March, she said yes and they broke up 2 days later. I'm so angry because if he didn't lie it wouldn't feel like a big issue. but he did, and i feel he is a coardly man who has deceivedd me