This is odd. Trying very hard to think of how tirz could cause one sided lower back pain twice after 2 injections, and really cannot think of anything that really makes sense, but just because some random person on the internet and I am guessing in advance, the ER doctor cannot think of an explanation either, does not mean it is harmless. It most likely is, but happening twice is enough for it to checked out, and it sounds like that is already happening.
I cannot think of any reason tirz could cause hydronephrosis either, which as I understand it is caused by a blockage to the outflow of urine from the kidney causing the kidney urine collecting system to swell up, would presumably be caused by kidney stones or tumours etc. Very hard to see how tirz could have any effect on it.
The things I would be concerned about are all very unlikely, but the problem is unusual. One is the tirz is not tirz, but mislabelled, cannot really see any of the other chinese peptides doing that either, but not impossible. 2 is it is some type of reaction to something in the tirz or bac water, if it was the bac, then it would be just as likely with any other peptide using the same bac water, and if the tirz , then a contaminant or endotoxin is possible. or 3 an odd very minor systemic allergic reaction, but not remotely typical. 4 just for completeness a nocebo reaction after getting an odd effect from the first injection. and most likely of all , no idea, but maybe something.
The question then is , is it safe to keep using it? It would probably be a good idea not to. Might be worth getting a totally different batch of tirz and trying a dose of that, but if it happened again I think at that point you might be tempting fate to keep using it even if there is no useful explanation for the cause. Are you very overweight? The tiny dose suggests you might not be, not being able to use it would be a more serious problem if you were using it to treat severe obesity and just starting at super low doses. But I guess you could just use reta instead, cross reactions to GLP drugs are rare.