Hi, I Have A Question About Intercourse. Me And My

Hi, I have a question about intercourse. Me and my partner have sex very often and at that time am very comfortable but I seem to have problem after some gaps. It happened a few times, now I am really fed up of the situation. When ever my partner goes away for a while and after he comes back and we have sex it hurts me really bad and I get really sore but it eventually gets better after some time. This has been going on for a while now and I really want to know why it happens.