Cat's tail went limp!

My cat usually goes in and out of the house regularly (he was adopted from the street and has some friends outside). Last night there were fireworks, so he came in quickly, and I noticed his tail was drooping and he was sitting very cautiously. He never wants to go under my bed covers; in fact, he used to be scared and avoid them, but this time he tried. He doesn't seem distressed or necessarily in pain; he even made himself comfortable on my bed (an unusual position, but okay) and started purring. This morning I noticed the same thing as yesterday.

My main concern is that a few months ago he had an emergency urinary obstruction and was prescribed a medicated food that was too expensive for us to pay, so we continued to give him conventional food, although it is a bit more expensive.

After that he disappeared for 18 days and returned malnourished and very thin. He didn't want to eat, although he still purred when I petted him. After a little less than a month, he recovered.

Should I take him to the vet? He doesn't seem to be hurt, the behavioural change is what alarms me.