My niece had the exact same problem and my sister took her to the hospital. The doctors took a blood test and couldn't find anything wrong, what they ended up telling her was to cut back on salty foods, that maybe her salt intake was to high. This happened about 6 months ago (my niece was 3 years old at the time) well as soon as my sister started watching how much salt she put in the food my niece was fine and it hasn't showed up again. So all I can think of is to cut back on your toddlers salt intake. Hopefully you find this helpful.
I just took my daughter to emergency on the weekend because both her feet were red, swollen and hot to the touch. She was so sore she couldn't walk on them. The doc at the emergency said she was having a reaction to the virus she had and that she may get hives. Sure enough, she got a few spots on her the next day. She's completely fine otherwise but her feet were sore again last night but not swollen and red.
We are following up with our family doctor tomorrow to make sure that it's not more than a reaction to a virus. She did have an ear infection the week before.
Something similar happened to my son when he was almost 3 years old. After 3 days, 2 trips to the urgent care and 2 misdiagnosis (the first was wrist sprain, the second was spider bites) we finally got transferred to the Children's Hospital ER where he was correctly diagnosed with HSP or Henoch-Schonlein purpura.
Day 1 was swollen wrist joint and hand swelling (trip to urgent care 1). Day 2 he was better. Day 3 swelling was back and he had red bumps (like numerous spider bites) on his ankles (trip to urgent care 2). Day 4 was a Monday we say his pediatrician at 3PM that day. By the time we took off his pants for the exam, his legs had so many bumps and were so swollen, they were almost unrecognizable.
HSP sounds and looks very scary but, the good news is that it usually resolves itself with no side effects and no medical treatment necessary. In rare cases, it can involve the kidneys so if this is the diagnosis for your child, you will likely be prescribed routine blood and urine tests during the active disease phase (about 3-4 weeks).
As it was explained to me by my son's pediatrician, HSP is an abnormal auto immune response, typically as a reaction a bad cold. It is not a common condition but most children who have it are in the toddler age range. This coincides with their social phases (touching other kids frequently) or when enter preschool for the first time and get exposed to new viruses and bacterial infections.
It takes a highly trained clinician to diagnosis this condition and since the condition is rare, many may not be familiar with it.
Consider suggesting it to your MD... it's worth a shot. Good luck!
After many trips to the Dr the only conclusion they have come to is that my son has a severe sensitivity to the heat and cold. The only thing I can really do is allow him to be outside for very short periods at a time...
Good luck to the rest of you, I hope your answer is as simple as mine.