It sounds like spotting which can happen at any time with th mirena, just means a bit of the uterus lining has come away and bled a little. Could be cause by over exertion (including sex) but lifting heavy things, overdoing the exercises etc.
It should settle again.
The other thing to be aware of is that Mirena lasts on average 5 years, which means it can run out sooner than that too.
If this spotting continues over a longer period of time (3 months worth of periods) then perhaps it is telling you it needs replacing.
The lining has been in the uterus for a VERY long time and like all blood things it will turn darker and darker over time so any spotting of dark sludge is old lining -if you get a fresh bleed then that is bright red as per usual period blood. If it is already darkening then chances are it is new-ish and got stuck on the way out the cervix for a while.
You could visit the GP to check for infections if the current spotting carries on more than a week. If it clears up then it was just a phase and may happen next month again of the coil is running out or might not happen till it does run out.
By which time your 1st proper period without the coil will be significantly dark sludge so don't be shocked that is quite normal.
It is why endo cysts are called chocolate cysts..the old blood has lost its oxygen and like scabs on the skin it turns brown when it dries up and gets old.
I have had a few of these spottings with mirena....nothing anywhere like a full period.
I have had two proper periods also earlier this year, a month apart, but just as I was looking to get the mirena replaced they stopped and the mirena woke up again and went back to work...thank heavens.
As for th sexpain issues that could have any number of causes, the main ones being scar tissue or adhesions on the cervix and vaginal walls inside and/or outside of it.
No cure for that aside from either taking pain killers before the deed, or abstaining and finding other ways to be intimate.
Mirena doesn't stop endo that is already in place, it will stop more endo from backflow of menstrual blood up along the fallopian tubes, but if you did have remaining endo before the mirena went in then it could still be active and now getting painful if it has reached sensitive nerves.
So the spotting is Normal for mirena, the pain issues are your body telling you something isn't well, and perhaps that does need to be discussed with a GP then a gynae as they may want to do more investigations in to the state of your endo and perhaps check you out for cysts too.