Make sure you always use a new needle! I've found that if I used a needle even twice, I get a bruise. My cousin who is diabetic showed me how to use my arm myself and it works great. Maybe you already know. Put your leg up on a chair while you're standing. Bend your arm and push it against your leg so that your fat from the back of your arm is visible. Then put the needle with your opposite hand in that fat spot on your arm.
I don't have a lot of fat either. I weigh 102. And I workout at the gym constantly so my arms are mostly muscle and bone. Everyone has a little fat on their arms, you just have to be able to push it all together to find a good spot. And I rarely get bruises there. I don't take vitamin k either.
Just a suggestion.