[Help] At a loss with my itchy/chewy dog!

Posting here to see if anyone has experienced a similar thing and has ideas?Our dog (about 1.5 years old, rescue mix, 35 lbs; we have had her since a puppy) is suddenly, since early Spring, very itchy. To the point that she is chewing patches of fur out on certain parts of her body (arms and legs mostly). At times she seems to nervous-chew, like when the cat is around. But also is just darn itchy in general. She doesn't chew enough to cause redness/break the skin, yet.We've been to the vet three times now for this issue. First time, they said it was just 'pressure sores'. Second time, I pushed the issue, and they did a skin scrape which came up negative. Third time, they said it is probably allergies, and told me to give her some benadryl.Here is what I have tried/am trying:--She is already on a monthly flea/tick preventative (Nexgard),--One benadryl per day, sometimes twice a day if she seems super itchy (seems to help, but only marginally)--Switched to grain-free food the past few weeks, as I read this could be an issue. This seemed to really help at first, but she's been REALLY itchy the past week or so - seasonal allergies are in full blast too right now, though, so maybe related?--Give her coconut oil/peanut butter a few times a week in a kong; and have rubbed a bit of coconut oil on her skin but she tends to lick it even more when I do that!--I've avoided bathing her to avoid drying out her skin.Any other ideas? Should I get a (fourth) opinion from a different vet?! I see those anti-itch sprays at the store, are those worth trying? I'm out of ideas and she is running outta hair!*Edited to add - I just called my vet, and they mentioned trying a trial of anti-depressants - is that common for itchy dogs?! via /r/dogs http://ift.tt/2rtq4Zs