First i am really sorry to hear that you have been sick all weekend and stayed in bed.
You should have mentioned why you were in bed - was it due to fever
or something else?
The knot and bruises (without trauma) you described sounds like spontaneous subcutaneous bleeding which can be due to low platelet counts or clotting disorder
There are many possible causes of low platelet count , but most common being viral infection.(usually dose not cause large hematomas)
And clotting disorder can be associated with genetic or liver
The arm pain
along with the spontaneous subcutaneous bleed can be suggestive of a serious disease and you need to go to the doctor to get your blood tested - Complete hemogram and peripheral smear , which will suggest further investigations.
I hope this will help you
good luck and get well soon