I have had the same symptoms you describe after taking many powdered herbs. It feels exactly like a bladder infection. I have tried the herbs at different times, but always the same reaction. I use herbs only from starwest, so I don't think additives are the problem. I would love to figure out what is going on because there are lots of herbs I know I would benefit from taking. When I take them, the pain is very intense. It is not possible to just keep taking them and still function.
There is nothing in ashwagandha though that should cause this. A spastic bladder can also cause these symptoms, but again the GABA elevating effects of ashwagandha should prevent bladder spasms as well. So I just don't see any way for the herb to cause these symptoms.