If the substance that is causing contact dermatitis is removed your symptoms will probably go away in less than 2 weeks. With treatment it will resolve even earlier. You should avoid scratching. Scratching causes a vicious scratch itch cycle which aggravates itching. Your dress could be the offending agent though not very likely as you have been using it for a month. Most commonly the offending agent is an or vegetable protein. Substances that can cause contact dermatitis include foods, metals, plants, soaps, cosmetics, perfumes etc. Thanks and best wishes.
If it continues to wprsen, then you will have to see a doctor. You have not taken benadryl. There is no set limit or rate for spreading. If you take that are likely to start improving but yes the risk of drowsiness is definitely there. However, if you have have constant itching you are not going to feel better anyway. Taking a cool water bath will also help. Try to remain away from all those things which you think you could be allergic to. When it's time to go to sleep you should take benadryl. Thanks and best wishes.