Sounds like a muscle strain. They can happen any time but your recipe of 30 days off followed immediately by a harder than average workout is a classic example of how to strain a muscle. Any time you have been off for a while you have to give your body a chance to come back up to speed. You can't just jump back into your training where you left off. I'm not saying you have to baby yourself for an extended period, but take it back a notch or two and increase your efforts incrementally. It is a mistake to try to make up for lost training by hitting it hard right out of the gate. As you are discovering, an injury slows you down a whole lot more than taking a few rides to rehab from your downtime.
Treatment for muscle strain:
First 48 hours
- ice (runners often sit in a shallow cold bath for 15 minutes every few hours to rehab muscle complaints, pitchers ice their elbows, etc.)
- compression and elevation are possible with extremities, not so easy with your backside
After the swelling has stopped
- alternating heat and cold therapy
- gentle massage and light stretching (don't overdo it or you will reinjure the muscle and have to start over
- incremental increases in use as tolerated
- plenty of rest and good nutrition
There is some controversy about the use of NSAIDs (non-steroidal anti-inflammatory drugs) like aspirin, ibuprofen, and sodium naproxen. They can reduce discomfort but some studies are showing that they may actually inhibit recovery from musculoskeletal injuries. Not a big deal for the average person going about daily tasks of living, but if you are looking to speed recovery and get back into training, you might want to minimize their use in favor of other pain control, like heat and cold therapy, or just sucking it up for a while.
IMHO, excessive use of pain relieving drugs contributes to a lot of reinjury and the development of chronic injuries by masking the signals that should warn you of overuse. Remember that pain has a purpose. Using OTC pain killers to allow you to workout hard while still recovering from an injury is like pulling the battery out of a smoke alarm and saying the fire is out because the alarm is no longer sounding.