Rats, sorry for the post pregnancy issues! I tell you, my feet absolutely were bigger after I had a baby. Your bones relax (we are preprogrammed to do this as we give birth to help with the delivery of a baby) and we put on extra weight. I went from a size 7 to an 8! I had to give away all my cute shoes! Also, in general, if we are a certain weight, our feet can also be a bit puffier and we wear a bigger size. Lose weight and this sometimes makes it better. So, I'd invest in some bigger shoes for now.
And plantar fasciitis-- oh my. I have this as well right now but only in one foot. I've had it since July and it flares up and is quite painful. I had insoles I put in to try and help and have now removed them because it made it worse overall. Feeling a bit better. Try lots of stretching of your calves, roll a golf ball on the heel (put the golf ball on floor and roll heel over it), try icing it and ibuprofen (if you can take that now that you gave birth).
The arthritis I am not sure about. Talk to your doctor.
Let us know how you are doing!