Suggest Treatment For Red Spot On The Leg

I was wondering if you could help me. I recently went hiking in Devon and when i came home i had a really intcy, what looked like, a spot on my leg. I just ignored it but then a few days ago when walking to pick up my brother from school my trousers were rubbing it and it was stinging. Again i ignored it but when i woke up this morning and it was bloodshot red and really sore. Any idea wat this could be?