I think it’s highly unlikely that supplements cause pains unless you can correlate the supplement you are taking with known side effects. Then again 750mg is a considerable dose. Maybe go down in the dosage, if quitting can cause problems.
Depending on the cost of seeing a doctor where you are now. I would highly recommend both seeing them and stopping the L-theanine. You ideally want to rule out all causes here and if it’s a simple fix then you don’t want to be dealing with pain for a whole month
I don’t know how much of a drinker you were or whether it was spirits, wine or beer but liver damage can be permanent after a time, so watching out for liver pain is key.