I had a cold during my fifth month and the doctor prescribed the same but made me choose between sudafed and regular benadryl. I opted to take benadryl instead (but only once during day and once during the night instead of every 6 hours). I think it's fine, just take it as recommended and/or less frequent.
I also got a humidifier and a vicks vaporizer in my room which really helped. Also, I propped my head up with an extra pillow to help me breathe. Just to let you know, the cold will lift after two weeks. You're bound to get better, although if your mucus turns yellow or green, you might need antibiotics so take plenty of rest and take it easy so as not to make your cold worse.