Benadryl will help with allergic reactions but probably not for your chest tightness. You need to go to your closest emergency room as soon as possible for chest tightness. It could be more than just an allergic reaction. You should see your family physician to let him or her know that you are having this problem.
If you are having an allergic reaction to macrobid and having trouble breathing call 911 immediately. If you are experiencing itching of the skin, benadryl can help with this problem. Call your doctor to let her know of this problem. Usually there is an alternate medication to use.