Not even sure - could you help????

My little Kitten Misty is a girl and she is about 4 and a half months old. She is meant to be an indoor cat because I live close to a highway and the other cats I have had got run over so I made the decision to keep her inside.

Well last night she got out and I don't know how and I didn't realise she was missing till the morning when I was in the bathroom and heard her under the house meowing. So i went and got her and she was acting all affectionate and purring heaps demanding a lot of attention and putting her bum up in the air.

I don't know if she has mated or even if she is old enough to go on heat. All day today she has slept and she is still acting weird as I'm writing this. She NEVER purrs or is this affectionate? What are your thoughts?