If these bumps are very hard and round and are regularly spaced from each other in a vertical line, then the most likely explanation is that, by pressing hard near your tailbone, you are actually feeling the bony protections off of the spine and tailbone. These can be quite prominent but are, obviously, just part of your normal anatomy.
On the other hand, of the bumps are soft and not in a straight line, then there are a few other possibilities. The most likely is that they are just simple cysts under the skin. Cysts can occur anywhere on the body under the skin, and they usually are not a big medical problem. However, one type of cyst, called a pilonidal cyst, occurs near the tailbone and can become inflamed and infected. For this reason, if there is any redness or pain over these lumps, you should seek immediate medical attention to rule out this possibility.
Regardless, I do recommend that you bring this issue up with your primary care doctor
. They will be able to take a look at what it is you are feeling and make sure that this is just a normal finding and not something that needs further investigation.