to people with older children. you maybe able to help

Bit of background. My toddler who is 2 and a half is hard as nails. Runs into stuff leaps off things always fine. So last Tuesday he just jumps over a tiny soft football. Immediately limps. Let him settle overnight and reassess in morning. Next morning. Still limping. Showing no signs of pain apart from the limp. Take him to a and e who won't x ray as they can't pinpoint where it hurts. They won't x ray full leg. Friday still no better take him to our local dr. He thinks he's slightly tender in left ankle. That's x ray and it looks normal. No we are a week after the initial limp. No improvement at all. At a loss. He looks peaky but no fever. He does have a skin infection that we are going to drs again about today as it hasn't healed with antibiotics and he's had it a month. I'll mention the limp again but feel like I'm hitting my head against a brick wall. To me a sprain should be healing by now. Anyone had this and what was the problem.