what is this?

I had to get my blood drawn at the doctors on saturday morning and by saturday afternoon there was a golf ball size lump where this bruise now is, super swollen and very very tender to the touch. the next day it turned into a gnarly bruise and it seems to get worse each morning. she had the tourniquet on my arm very very tight the entire time she took my blood and didn’t put much pressure on there afterwards.

so

1. what is it?

2. what caused it?

3. how serious is it? like is there any reason I should go back to the doctors for this?

4. is it normal to have dull, residual arm pain after something like this?