Bleeding after sex
BLEEDING AFTER SEX IS NOT NORMAL. IUDs do not cause bleeding after sex. If you get bleeding after sex, you need to see a doctor, and that's not a maybe but a must. ETA: I was referring to actual bleeding, rather than spotting, though it's wise to check out spotting just in case. Both copper and Mirena IUDs may cause irregular bleeding, for the first few months in particular, but that's not the same as distinctive bleeding after sex.
Here is a list of the top ten causes of bleeding after sex. I've just had a year of bleeding after sex which was caused by a cervical polyp, #5 on that list. It was my third polyp, and although the doctor says it's all coincidence and that some women are just more prone to them, I do suspect that the IUD may be a factor, since two of the three polyps occurred when I had an IUD in (with this one, it happened within a month or two of the IUD insertion, which meant I started getting bleeding after sex not too long after getting the IUD). Anyway, apparently it hasn't caused any problems and I'm still perfectly fine to use an IUD.
Here is a page for medical professionals which discusses how to deal with the various problems which may arise with IUD use, including bleeding problems. Note that bleeding after sex is not listed as a normal sequel to getting an IUD, but that it is listed as a possible sign of PID, which is taken very seriously indeed.