So sorry, honey! You need to get to the doctor. The truth is, it is probably no long just heat rash. It's probably now systemic and can end up spreading and causing you a lot of trouble.
This happened to my husband, though heat wasn't involved. He got a spot on his leg. (Don't know what it was from.) That was on a Thursday. By Sunday night, it had spread all over his leg and was driving him up a wall. He was supposed to fly out on Monday afternoon. He didn't know what he was going to do - he could hardly sleep, it was so bad.
I had a relationship with a dermatologist who had seen me and my kids. I called that office and explained what was going on and begged them to take my husband. They did. I took him down there, the doctor said that it was impossible to tell what had started it, but it was in his bloodstream and could end up being serious. ALL OVER 4 DAYS, Mama! Scary! He gave him a shot that he said would stay in his system for 6 weeks. He gave him powerful oral antibiotics, and some powerful prescription medicine in cream form to put on it. He also told him that if he didn't get better or got worse, to go to the hospital where he would be visiting and have them call him. My bet is that he was thinking IV antibiotics.
My husband DID end up getting better, though it took a few days. He went on his business trip and did not have to go to the hospital. Now when we see a little spot on his leg (and we have), we treat it immediately. We also, per the doctor's instructions, switched to Tide or All Free and Clear and he uses Cera Ve lotion on his skin (the dermatologist sells it a little cheaper than the drug store). We never take this stuff for granted now.
Please get to the doctor - a dermatologist if you possibly can. I don't want you to go through this like my husband did.
Dawn