Hmm... I've never heard of just the feet swelling, but it could be? If you are worried you had a problem with peanut butter, heres something you can try.
Sit down resting for about 15 minutes and then take your pulse.
Eat something with peanut butter in it (but stay sitting, don't get up and move or it wont work)
Take your pulse in another 15 minutes after eating the peanut butter.
If there is a noticable increase (a few beats per minute is normal, anything over 10-15 is what you are looking for) you may have an allergy to it. I'm not entirely sure why this works but it does. It has something to do with the body responding to an allergen so your pulse rate increases.
Or you could go to your doctor and get an allergy test. Nut allergies are one of the worst ones, so you want to make sure if you have it so you can avoid it