This is the main problem caused by coccydynia (the tail born pain). How painful it is also depends on the design of your chair and the padding. The increased pain and sensitivity caused by having to sit for a long period may continue for days afterwards.
Sitting to Standing:
This symptom is particularly interesting, as Doctors found that all of the patients who had this particular symptom had a coccyx that partially dislocated or moved abnormally when the patient sat down. The reason for the pain is thought to be that there are various muscles you use for rising which are also attached to the coccyx. When you go to rise, the muscles pull on the coccyx, pulling it out of position if the joint is damaged.
To avoid this pain, try sitting leaning forward a bit and hollow your back a lot. This got rid of pain going from sitting to standing.
Anyway, please meet your doctor at your earliest.
Get Well Soon!!