The symptom usually suggests a calcium deficiency or an injury. It sounds like your frog gets calcium though, is it calcium+d3 or just calcium? Vitamin D3 is needed to metablize the calcium, but UV lamps should provide d3. I wonder if it is enough considering how much time pacman frogs spend down completely submerged.
Injury wise, I don't know what could have happened to hurt the frog. Pictures would help.
(Apologies if there alteady are some, I'll check on my pc later. Mt phone's data is too slow to load pictures.)
Also, where did you get the frog? You said you had it for about a month, but it is 3 months old. It is possible whoever had the frog before you didn't properly give the frog supplements.
I hope this helps.