What can this be? (urgent please help me! thanks!)?

so for the past 1 and a half years I have had a penny-sized, pretty flat, very palpable, painless, somewhat squishy (if i press very hard...it's not rock hard though) "thing" in my crack about an inch down from the top of my crack and slightly to the left of the spine. it came during running season, and when it first came it hurt a lot but then once it reached the size it is now it stopped hurting. it hasn't grown since then that I have noticed, though recently (the past month-ish) i have been touching it a lot so maybe it got a tiny bit bigger but not very noticeably. the fact that it's flat is what confuses me, and painless, and my first thought about this "lump" (which btw is NOT externally visible at all in any way! no redness, or sticking-out-ness). sometimes at night it's slightly itchy, but that's only been recently so i think that's probably because i keep touching it. i didn't touch it at all for the first like year i had it so what could it be? oh and this summer another one formed but it's much, much smaller and about an inch lower than this one and it's still very small (like the size of a SMALL pea). so what could this weird flat sorta squishy thing be? i do sit on my tailbone there often and also i'm 16 years old and a girl and i'm average weight (slightly on the skinny side) - i read that it could be lipoma (some fatty thing) but could it still be one if i'm definitely not fat? lol. okay please help me! basically what i'm hoping it is not is that it's not cancer aaah okay please help!!! thanks!