If you can feel it inside and outside the seat tube then it probably is a crack. It's a pretty easy lug to replace so it doesn't need to be terminal. I'd keep riding and see what happens. If it does spread than you can drill a hole at each end of the crack to stop it propagating as an interim solution.
If you can feel it inside and outside the seat tube then it probably is a crack. It's a pretty easy lug to replace so it doesn't need to be terminal. I'd keep riding and see what happens. If it does spread than you can drill a hole at each end of the crack to stop it propagating as an interim solution.