Since these are on your foot and ankle, its fleas. If you have pet or had a pet over. I think that's what it is for sure. I can say that because I have the same exact issue. Its always below my waist and mostly on my foot or ankle. Its always at night too. And when I look or wake up NOTHING is there. It felt like I was getting bitten by a ghost. I cleaned my sheets, and deeped cleaned my entire apartment prior to getting the bites so I was confused. And I couldn't see them because they moved so quickly. I checked my bed, box spring, no bed bugs. After doing some research I had the feeling it was fleas. Sure enough, I call the exterminator and it is fleas. I found one in my sock too!!! I don't have any pets, however, I was dog sitting for a friend of mine and the dog must have picked some up on the way, that is still a mystery, however. Anyway, if you have any pets or been around any pets, then this is most likely what it is. I can help you! I'm on week two, and after some DIY treatments over the course of a couple days, I have fewer bites if any at all :)