I have this happen all the time and so do most ppl, I promise you. When we worry about a specific disease we are on the "look out" for more symptoms and warning signs and if you go looking for something, often we endnup finding something and somehow linking it.
When I thought I was pulmonary m embolism, I hadb"lung" pain, when I thought I had a blood clot and alreadybhad leg pain, I was looking for warmth in my leg and yup it arrived. When I thought I had lymphoma (yup still getting over this one), I was looking for itching and you bet that symptom arrived as well. On and off and is now gone.
Bruises that are unexplainable happen all the time. In fact, now that you mention it, I have at least 3! Don't worry. Leukemia especially is easily detectable in blood ( something will be off) and you would feel awful. You are fine.
Something my doctor told me today was that I have many years to worry about my healthbwhen I'm older when it becomes a legitimate concern. Right now, we are healthy and should embrace it instead of finding issues someway, somehow.