Something similar happened to me recently during my usual weekly shot in the love-handle area. There is muscle there. It took a week to fade. I am moving to a shorter needle. Losing fat brings its own set of issues.
Weight loss has left my glutes with less padding than they had a month back. For that compound I stick with a half-inch needle so it goes deep enough to skip the red welts. Last night I pinched the skin and still didn't think I reached muscle, yet I woke up with pain in the glute that ran down the leg and showed I had gone too deep. I don't have tips for anyone else, but the takeaway is to pay closer attention next time because that muscle ache is no fun.