I think you should go to the doctor, because it's highly recommended that any symptoms like spotting do get checked out.
t could well be PCOS but that would depend on an ultrasound/blood test for a firm diagnosis, especially as you say you don't have the other symptoms.
If it is PCOS, as the person above says it really shouldn't be a scary diagnosis, and the dr if good might be able to help you out and check that other issues don't arise (it can be bad for your health if your periods reduce further and you don't get them for months at a time). I'm doing SW too and find that it has worked for me, given that carbs and sugar are naturally limited. I'm trying to stick to wholegrain carbs now and limit my carbs portions and sugar to keep my blood sugar stable. There's lots of advice online about the best diets for PCOS. Losing weight is likely to help your symptoms but it is not a cure-all unfortunately.
Likewise if it is to do with your ED then I'm sure the doctor can help you out with nutrition advice or a referral - for example making sure you're now getting all the right kinds of fats and oils you need for your hormones to work (this is possible on SW through hexBs!). Don't despair, I had a friend who was at death's door with anorexia while we were at uni, and she got pregnant by accident a couple of years ago, she and her little boy are very healthy and happy now, so once again that is not always a life sentence!
Anxiety can be hard but could you try and tell yourself that not knowing is only going to make you more worried? Good luck.