I get this pressure feeling in my ankle, it's not exactly from the sides, it's from the back, near your heels. What is it?

Achilles related. The area of the ankle behind the heel is where the achilles tendon attaches. If you are concerned with this area, you may want to have the foot/ankle specialist check it. Conditions like achilles tendinitis and retrocalcaneal bursitis can cause swelling which can feel like pressure. Get checked!
Achilles. Hard to say exactly what may be causing this with the info given. This is the area where the achilles' tendon attaches. There are several conditions that could be the cause; insertional achilles tendinitis, bursitis, periostitis of the heel bone.... If it has just started and is in both feet i would suggest assessing any increase in activity, unusual activity or shoe type.