Internal storage full, even though space is definitely available

As soon as I switch on my phone, for the first few seconds, storage is shown at more than 30GB showing more than 29% available. After a minute, all the space disappears and I start getting loads of popups telling me that all kind of services stop working. I check storage space again and I find it full, to the least byte. I managed to remove a lotr of apps to make sure that I had enough space, but whatever size is available, it keeps filling it all up within seconds. It is not letting me install any new apps - ram checker or anti virus, but after it downloads, it states that no space is available. Any ideas? Thanks

Chris