I've heard of this happening though I'm not a doctor. Apparently its not uncommon for amputees to get sensations where the lumb used to be. I would speculate that sexual arousal kinda perks your nerves and your brain up to be more aware. I guess it would make sense. When we're aroused we respond differently to being touched in different places so it might be extending to your amputated leg.
It sounds normal and in line with what I've read and heard but again, I'm not a doctor or an amputee so I can't really say for sure. If it bothers you, if it freaks you out, or even if you're just curious I would suggest contacting your doctor or the doctor who performed the amputation just to be sure nothing else is going on.
I am not an expert/doctor but I think that the swelling can be influenced by the changes in the process of blood circulation during arousal and the tingling sensations may be caused by neuronal impulses whose courses and functions might have been altered a little bit.
But I hope that someone would be able to provide an answer which would be more qualified and specific than my "speculation".