Haha, ETE, this is crazy! Just a few weeks ago the exact same thing happened to me -- out of nowhere, I noticed a line of yellowish bruises on my forearm! Just like you, I freaked out and was convinced I had leukemia. Guess what? I didn't, and neither do you. If you had leukemia, you'd be experiencing night sweats, weight loss, fatigue and a range of other symptoms. And just as a side note, bruises from leukemia don't just appear out of nowhere. They form just like regular bruises -- the only difference is they form more easily. If you had leukemia, you wouldn't just be bruising in one spot -- you would be bruising everywhere, every time you hit a table/chair/hard object.
My advice is to just to calm down, relax, and try to stay off Google! Trust me -- you'll be okay. I think forearm skin must just be particularly fragile, if we both had bruises there! Mine disappeared in a couple days, but it can take a few weeks in some cases. I hope this helps and I hope you feel better. :)