hate to break it to you, but that sounds like an alien is incubating / living in your body.. probably injected from a tattoo needle, starts to grow through your skin.. like in the Sigourney weaver movie...
however, if you think that s not the case, it might also be just a simple skin irritation, clogged pores from either dirt or too much sweating / aftercare lotion. does it get excessively hot where you at? could be an infection / staphlococcus... an ink allergy... I m shooting in the air since I cant see what you speak of, but these may be something you may want to research or look up.
a photo might do wonders - not sure if it s on the tattoo as well, or just outside of it.
best advice, don't trust anyone on a forum... speak to a dermatologist. hope this helps.