Well, not riding for a few months is really not long enough to lose that much muscle, is it? I mean, assuming that you were still active?
I was bedridden for several months, then had a very long/slow recovery. Before riding again, I worked mostly on my inner thigh muscles (I used to have an instructor who called them your "stick to the saddle" muscles) by pressing a ball between my knees (a medium-sized playground or soccer-sized ball).
I had also lost a lot of upper-body/back strength, so I did modified "push-ups" by standing in front of an open door, with each hand on each door jamb (sides of doorframe), with my feet 2 feet away and pushing against the doorframe, like a push-up. You can play around with the distance of your feet and the angles of your arms (elbows down vs out to the sides) to work different muscles. you can also hold the position to "plank".
Good luck and enjoy getting back into riding!