you are NOT alone! immediately after having my daughter i was in serious pain in my tailbone. not so much my lower back, but then again, the pain in my tailbone was sooooo bad that i couldn't feel anything else. was your daughter a big baby? my daughter had to be vaccumm extracted and she was so large that the vaccumm came off her head once and they had to re-apply it. they finally pulled her out of me, but in the process my tailbone broke. before i was even stitched up i told my doctor that my tailbone was killing me. he said that i may have broken or bruised it. i couldn't tell you if i had any other post birth pain because my tailbone was KILLING ME. i couldn't sit up to breast feed, i couldn't lay down, i could hardly walk. there was NO comfortable position. i totally understand where you are coming from. the next day when my doctor came in to see me i told him about how much pain i was in. the first thing he said to me was "your tailbone will take up to a year to heal, whatever you do DO NOT have surgery on it". i knew better than to have any surgery as my grandfather had tailbone surgery and he said that was the worst mistake he ever made. my doctor did say that tailbones take the longest to heal and not to do much activity. well, heck, that wasn't possible as every movement hurt.
so i would say to you, stop doing those exercises. listen to your body and whatever position (i know there aren't any) is the most comfortable try and stay in. riding in a car was a nightmare for me! i also had asked for one of the inner tubes to sit on, but the nurse said no because that would have seperated my tushy and cause my stitches to pull apart, but you are past all that so you might want to think about an inner tube to sit on for some relief.
my daughter is now 2 and to be honest with you, i didn't feel relief until she was one. also, to this day, when i sit too long in a hard chair, or a long ride in the car or when i do sit ups my tailbone will start to ache. NOTHING like what it use to feel like so i am happy to be to this point.
if you have any questions or want to discuss this anymore feel free to contact me. all i can say is try not to put any more trauma on your tailbone. any exercise will only aggrevate it even more.