Because it only happens in a certain spot in 2 rooms of the house (both at the back) I have been experiencing this for the last few months but never anywhere apart from where I have just mentioned, it may sound paranoid but I cant think of any reason why this only happens in those 2 rooms.. I have had gas checks done, also someone suggested mould spores in the air but although is a small amount of mould growth in one corner of my kitchen I cant see it causing that sort of problem. I drink occasionally and I dont take drugs..