H.G.
Sounds like a nasty combination of allergies and walking! :( My son tends to have a couple of bad weeks with sleeping whenever he hits a major new milestone, and it's always just about waiting it out. (And getting no sleep!) But with your little one, that cough sounds like it's making everything so much worse! Talk to your pediatrician about maybe taking her to an allergist. She's too young for Benedryl, I think. But a pediatric allergist would know what to try!
And of course 3-4 hours is way too long to let a baby cry! (Your instincts are right on target here. Keep following them!) Her naps may also be changing, which can make everything even harder. Maybe try to put her down for one nap in the middle of the day instead of two short ones? (Just an idea- who knows if it will work!) I know a bunch of 15 month old kiddos who take one long nap a day.
Good luck! Rest whenever she does!