Can you show a photo of the back of the bike - Start with it in one of the smaller gears on the rear cassette.

Also, just checking you've not put a new chain on. Or even taken the chain off for cleaning / waxing and then replaced?

Also, can you check the tension screw that points backwards out the back of the bike and is just below where the rear derailler is bolted onto the dropout. Very very rare, but if you've been using a lot in the small cassette, notably on a trainer, that screw could have vibrated a bit loose. And then when replaced the derailler perhaps that wasn't set correctly, mimicing the old one. Apologies if you did go through this, but in the spirit of trying to assist it's good to close out some of the 'obvious' first.