Yes agree with Shelly, definitely too soon to be riding your horse.
You should have really left it for about 3 months.
Nothing you can do now, but just take it easy and see your doctor if you have any problems.
You are just over the six week point and that is the minimal time it takes to start regaining normal activities. I am assuming your doctor already gave you the go ahead to return to normal activities since you attempted this activity. Even if that is the case, there isn't a "all ok" date since everyone heals a bit differently. The key is to not push yourself in any activity that strains your abdominal muscles until they fully heal. They have all been cut and you have healing that needs to take place on the inside as well as outside. If you do too much too soon, you can increase your chances of getting a hernia. A hernia post-surgery often results in the need for another surgery which sets you back far longer than it would to have waited.
My opinion is that I would not push and wait a couple more weeks before riding again. I think you should also call your doctor to let him know of this change. He may want to recheck you sooner if your pain does not subside in a couple of days. It sounds like this was too much too soon. Try some ibuprofen and heat packs. If it doesn't help and the pain does not get better, then it is definitely necessary that you get in to see your doctor right away. I hope you feel better soon!
Shelly