False negatives on pregnancy tests?

i'm just curious if anyone has had any experience with this or knows how common it can be. i've had a few friends who have said things like they never showed up positive on a urine test, only during a blood test and another who didn't show for three months with a urine test, ect. i'm over two weeks late now (usually have a pretty normal cycle, with terrible cramps and everything) and just took the test a couple hours ago. i plan on taking the other one when i first wake up since apparently your hcg levels or whatever are higher in the morning then later in the day. i had a uti a week or so ago, i was taking azos, any idea if that could effect it? or how long i should wait to take my next urine test? i have a doctors appointment coming up and plan on requesting a blood test if i still haven't started. i've just been feeling off, my body feels different. my / have been unbearably sore for weeks, for awhile my where actually itchy for awhile! i constantly feel exhausted and will randomly get SO sick. for awhile i was getting strangely dizzy, headaches. i don't know. i know what i really need to do is be patient and wait and what not, i'm honestly not even that stressed, just curious. for some reason i just have my doubts with the test. anyways, thanks in advance on any information or experience you can throw my way : )