I have a black/brown line at the top of my tooth I think I had a filling there a couple of months ago. What can I do? (photo)

The picture is really fuzzy and hard to make out for sure what is happening.  That being said, it could be stain buildup at the margin of the filling.  I would suggest you return to your dentist for eval.  It may be something as simple as a quick polish.  Certain foods, drinks and smoking can cause stains easily around the margins of the filings and make them stand out.

I would recommend going back to your dentist and having it repolished.  Bondings do stain with tea/coffee etc. Your dentist should be able to remove the stain for you and polish the bonding.