Quoting Supafly★:" That's not a tooth. My LO had the same exact thing around that age but it went away. I have no idea what it is. "
Same with mine (at 5 weeks). It was a little smaller then a regular tooth and I would have thought it was just my imagination (or faulty memory from lack of sleep), but the dental hygienists saw it too
He was so fussy, I was sure it was going to be colic. It got much better after the tooth (or whatever it was) poked threw.
I'm not sure when it vanished. But I think his first tooth came in around 3.5 months.