Girl stays at boyfriends house

My partner of 7months recently got drunk with a girl mate and she stayed the night. 40M and 26F they’ve known each other for years.
I think I don’t have anything to worry about but my brain is still going overdrive and has sooo many questions.
I have expressed I’m not comfortable with their relationship before but that’s because of prior things that just seemed a bit off to me. I also wasn’t aware they were ‘close friends’ when we first got together. He has other ‘close’ girl mates I knew about from day one and I’ve hung out with them and think they are great.
A few weeks ago I knew they had a conversation about getting drunk together on a day I wasn’t around which I questioned as to why they would when I’m not there and why we couldn’t all hang out and get drunk together. Anyway they decided to get drunk together just the 2 of them at his house.
He didn’t tell me about it until I went round and asked some questions, I then later found out after asking more questions that she had stayed over.

He said he wouldn’t have mentioned it if I hadn’t asked but would always tell me the truth. He also tidied up before I came over because he didn’t think it looked good. She apparently asked during the evening if I would be pissed off which he said I probably would be ( can’t help but think they thought fuck it and laughed)

Am I being immature for not liking this situation? He said if it was one of the lads I wouldn’t question it. He’s told me I have nothing to worry about and it was completely innocent and that rather than her walking home he let her stay.

I feel like I’m going mad my brain is thinking all sorts of things.

I guess I want to know if I my feelings over being worried is valid?