Questions about anal sex

I'm far from a prude but anal sex had never been anything I'd been remotely interested in, the whole idea just didn't quite sit right with me. My fiance had brought up the idea of us trying it on a number of occasions in the past but it had never been a particularly big deal & I was always able to just laugh it off & that was that. Last week though, we were fooling around in bed & it was clear he wanted anal & I decided not to stop him from taking that route so to speak! I figured if this is something he really wanted to do then I should let him & I suppose I'd be lying if I said at this stage I wasn't a tiny bit intrigued; the time just seemed right. However, it hurt - a lot. I cramped up the whole way through but kept with it as I assumed it'd go away, which it didn't. I didn't want to say anything as he was clearly loving it & when he came the pain did subside as we orgasmed together. He loves it so much that we've done it 4 more times since then. My main problem is the pain isn't lessening the more we do it as I hoped it might & on the mornings after I'm finding I get the runs & particularly uncomfortable cramping. Is this normal & will my body be able to adjust to it or is it a sign that I need to stop this altogether? I would hate to completely stop something that he gets so much enjoyment from, he does so much for me that I feel I should give a little back, but if my body is reacting badly to it then I really don't have a choice. Any advice/tips about how to make this a more pleasurable experience for both of us would be welcome (if that's an option)!

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