What Could Yellowish, Dark Stinging / Burning Spots On The Back Indicate?

Sorry if my english is bad, i m Dutch (14 years old) I have a little problem, on the skin of my back. I first found out just a few minutes ago. I went to the living room because i had a stinging pain in my left armpit, and while I was typing this my right armpit also began to ache. probably just a little infected sweat gland. I ve had this before. But now, the real problem. I did a few sit-ups in bed (because if i do it on the ground my ... lumbar curve (Spine Anatomy site)? hurts) Then i went to the living, like i told U before. And when i lifted my shirt so they could look, they saw something on my back when i turned to walk away. There are/were dark yellowish on each vertebra (on the part of the spine that sticks out and that you can see with meager persons, like me :D). They don t hurt or anything, only a small burned feeling if i rub over them. So... long story short: I did a few sit-ups in bed (about 30-40, supporting my neck with my hands when my neck got tired , saw darkish yellow spots on each vertebra or bulb or whatever, They don t hurt or anything, just a burned feeling when i rub the bulb . So, what is this? When does it go away? Is it bad/normal? What s the cause? Thanks in advance Again, sorry for bad english and if i misused the terms, i just quickly looked on the internet to see what it is called in english