It's most likely the the nut. Why it wasn't giving you problems before but is now is anybody's guess. The important thing is there are some really easy quick fixes.
Quick on the fly fix - Assuming your nut is black, just slip a small piece of electrical tape under the string in the offending nut slot. At two feet away no one will ever know its there. In fact I did this on one of my most heavily used basses that had a white SG nut. The tape (in this case it was white medical gauze tape) was put on about two years ago and until this post I had forgotten all about it. Works perfect and no one has even noticed it. It serves its purpose until I get around to having a new nut installed.
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Alternate fix - build up the slot with some crazy glue. Obviously this can get messy and you might not get an exact color match, plus you'll likely have to touch up the slot with a nut file to get it to the right depth.
Official fix - contact your luthier and have him install a new nut, level and dress the frets if necessary and give the bass a full set up.
Good luck!