Never really had this problem before until today, sometimes i click my PF bookmark and it takes me to the main page but not signed in, now if i click ANY part of the site like general section then im signed in and ive not even typed anything

I should always be signed in like i normally am, but today its decided to log me out everytime i leave, but as i said all i do is just carry on using the site and it signs me in............... odd i thought, so just letting the relevant people(s) know incase there is a problem