As CSG2306 said above, it could be an allergic reaction to a chemical used by a local farmer, or worse still, a reaction to something someone is 'dumping' in that particular area (used cooking oil, engine oil and excess weedkillers are sadly dumped in open ground more often than people assume!)
Rubbing her back along the furniture sounds like a different thing altogether however. At 18 months your labrador bitch is noticing dogs 'hormonally' when you're out for your walks. Don't forget, dogs can smell bitches (and vice versa) upto eight miles away if the wind is blowing in 'the wrong' direction. Rubbing their back along furniture is commonly associated (in both dogs and bitches) with the pack mentality of scenting, i.e. an attempt to try and leave their calling card at every given opportunity - anywhere and everywhere!
Aside from taking care not to walk her where oils and powders could have been dumped regarding the first issue, you could try putting a sulphur cake in your labradors drinking water (usually available cheaply from pet stores or not so cheaply from your local vet). This acts to cool the blood, especially in warm weather, and also reduces the urge to mate. If you want to follow the problem up, please get in touch at thek9mind at hotmail dot com.