Hi.. Welcome to HEALTHCARE MAGIC.. I have gone through your query and can understand your concern.. As per your complain it seems like an allergic reaction to the toxic chemical injected by the sting of the insect under your skin.. It has led to inflammation that is leading to a raised red and slightly painful and itchy welt over the chest.. I would suggest you to take anti allergics like Allegra or Benadryl for relief.. Take anti-inflammatory painkiller like Ibuprofen.. You can also apply steroid ointment over the swelling as it will help in relieving pain,itching and redness.. You can do cool compresses over it.. You can also apply calamine lotion as it will help in soothing the skin.. You should avoid scratching the area when it itches to prevent infection and increase in inflammation.. Hope this information helps.. Thanks and regards. Dr.Honey Nandwani Arora.