A couple of thoughts:
1) Compared to other folks experience here (and mine), jumping from 7.5 to 5 seems like a bit much once you were at/near/below the physiological dose (which can be either 10 or 7.5 depending on the source). At this point, it would seem to be desirable to determine whether the pain is being caused by PMR, or another condition. That needs to be diagnosed by testing...an ESR for starters.
2) Drug interaction checker suggests that it can be problematic and even dangerous to take NSAID's such as Advil while on prednisone, and suggests that that should be done only with MD supervision. There can be increased risk of intestinal perforation by taking both drugs simultaneously, as they reinforce the toxicity of each other. I am on prednisone 8, and my doc only approved concurrent Tylenol, which evidently does not have the side effects of NSAID's while on prednisone. I have been tapering only 1 mg per month since 10 mg and have not had any pain issues at the level you are experiencing.
3) Because of 1 and 2 above, and because everyone does not have the same reactions to the common drug therapies available, please get your doc involved ASAP.