Hello and thanks for the query.
The most common causes for pain under tailbone and specifically on sitting are
Pilonidal cyst/abscess/ sinus
and coccydynia
While coccydynia is due to the pain and inflammation in tailbone,pilonidal cyst causes pain and discomfort when the cyst becomes infected and becomes abscess.
Chronic pilonidal abscess results in pain and drainage of fluids resulting in the formation of pilonidal sinus.
Consult your doctor for the examination and diagnosis of your problem.
Once the diagnosis is made,treatment becomes easy which will depend upon the cause found.
The treatment ranges from antibiotics to antiinflammatories to hot fomentation and to surgery.
I hope to have answered your query however you may revert to me for any other query.
Please close the query if there is nothing to ask any more.
Best of luck 
Thanks

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Hello
Although there are many ways by which you can injure your coccyx bone but in your case,pregnancy seems to be the cause.
Giving birth is thought to be the most common cause of coccydynia.
The child birth can cause the muscles and ligaments around your coccyx bone to overstretch.This results in permanent altering of these muscles which no longer can hold your coccyx bone in position.This leads to coccydynia.
Treatment consists of Sitz bath hot fomentation and antiinfammatories if pain and discomfort is mild to moderate however in chronic cases like yours,injecting medications into your lower back may help.
These injections are
Corticosteroid injections
Ganglion impar nerve block
Sacrococcygeal joint injections

Surgery for coccydynia is very uncommon and is reserved for those cases where all other treatments have failed.
Thanks

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