This doesn't look like a typical spider bite, but is certainly an allergic reaction to a bite or other puncture wound.  Without you capturing an actual critter, it will be impossible to determine what you are reacting to.  I have seen similar reactions to bee and wasp stings, bed bug bites (which are usually only found where the skin is exposed at night), flea bites, or assassin bug bites. This kind of round, red reaction with a hard center is also known after a tick bite and such a reaction can be the early symptom of lyme disease.  You didn't mention if you have any pets which can pick up fleas or ticks outside (even if they are treated), bring the pests in and then the pests can drop off to bite human skin.  Having the bite in a same location is also very strange which may indicate that there is something in your mattress that could be sticking you during your sleep. It is possible to have an allergic reaction to being jabbed by plastic threads, metal parts or wooden splinters that can develop in mattresses. 

Since this is occurring in the same area of your leg at regular intervals, you should have it checked by a physician and possibly get your blood checked for Lyme disease antibodies. This disease can rarely cause re-occurring symptoms at the initial tick biting site.