Get an X-Ray to see if you have a bulging disk . I have degenerative disk diease L4 and L5 . No heavy lifting 25 lbs, Golf , running . Lite stretching Good Luck Joe
As well as the above, it could be that you were sitting too hard on your sciatic nerve and putting pressure on it where it travels under the gluteal muscles. This should get better within 30 minutes or so of taking the pressure off and will not happen off your bike.
If you have back pain and get numbness both on & off the bike, see above postings.