How much Benadryl can I take in one day?

39 weeks. My midwife gave me the ok on taking 24-50 mg every 4-6 hours for PUPPS. I took 2 around 6 pm, went to sleep around 8 and woke up at 11:30 scratching my skin off, miserably itchy once again and wide awake. Should I take one more? I will most likely need to take it again in the morning and throughout tomorrow. But how much is too much? I haven't taken any meds at all this pregnancy. But these PUPPS are really ruining my life. How many times can I safely take Benadryl in a day?