Hi Steph.
I agree that there is always something to worry about. I was only diagnosed with stage 1 endo, but unfortunately got severe pain before and during my periods, as well as loss of old blood sometimes up to a week before my actual AF.
I don't feel that any dr really can give definate answers about endo. My periods were never as painful after I had my son so I held onto the fact that maybe pregnancy had got rid of it. He's now two and about 6 months agao I noticed loss of blood starting to appear about 4 days before AF again.
Miraculously, I am now 13 weeks pregnant naturally (found out the day after I had confirmation letter of starting IVF again!) but have had much spotting - particularly old blood. To begin with, they did see implatation haematoma, but after about 9 weeks that appeared to have gone. I had 3 weeks of nothing, than last week it started again. I've also had pain, particularly where I used to be sore around AF time and am convinced that the stretching and moving could be knocking away old blood - at least this thought is keeping me vaguely sane...
The upshot? - who really knows! Like the other girls, I think it is most likely stretching and moving - think what's going on in there!

You will start worrying when it goes away!!!! - but don't worry - we always find something else to fret about - hee-hee.
If you are really fretting, speak to someone - that's what they are they for. You take care and enjoy the next few months.
Zoe