This is a good question. It is important that you make an appointment with your primary care doctor
to have your rash examined as it is difficult to know what your diagnosis is without seeing what it looks like. However, from the history you have provided it is most likely an allergic reaction to what you were exposed to at the park. This is even more likely if the rash you have is characterized by varying size raised welts that fade as new ones appear. Antihistamines such as Benadryl or Allegra can sometimes help with the itch. It is also possible that you are suffering from contact dermatitis from what your skin was exposed to. In this case the rash would be just in the areas where you came into contact with the sticker weeds. This rash often needs steroid cream such as hydrocortisone cream to improve. Again, as it is difficult to make a diagnosis without seeing the rash it is important that you make an appointment with your doctor as soon as possible to get a correct diagnosis and appropriate treatment. In the meantime, it is very important that you do not scratch at the rash as this can lead to infection.