>A spider bite pierces the skin and chemicals contaminating or injected into the bite site can cause the breakdown of tissues in susceptible individuals. To the untrained eye, this can appear to be a bruise, but it's not really.
>Blood vessels will normally become more permeable and this allows very small amounts of blood to escape into the tissues along with plasma.
>Although it can appear to be a bruise, it will normally, but not always have a focal point where the bite was.
Source: https://www.quora.com/Do-spider-bites-cause-bruising
I think you should go see a doctor just to be sure. If it's a spider bite and it's left untreated, it'll get worse. If it gets worse in the next few hours/days, you can be sure that it was a spider (but I wouldn't wait until then).