MY first thought when I read your symptoms is that you may need to see a general practitioner/family doctor. You could have any number of other issues that aren't pregnancy related. From infection, to hormonal imbalance to a tumor or cyst. While many of the symptoms you described CAN be pregnancy symptoms, they can also be that of many other things as well.
I would also like to clarify something that you mentioned. Previous ejaculation does NOT have to occur for there to be sperm in the pre-ejaculate. Pre-ejaculate is the SAME THING that comes out when a man ejaculates it is just released in smaller intervals to aid with lubrication. Therefore, there IS Sperm in pre-ejaculate.
The vast majority of the women on "I didn't know I was pregnant" only took one or two tests...not 20+. It's been known to have a negative blood test and positive urine or vice versa but I've never heard of BOTH being negative after having so many at different intervals.
You are NOT pregnant. Speak with your family doctor about other possible conditions that could relate to your symptoms.
But in just a few short weeks you'll definitely have an answer as to if you are or not, as you'll be giving birth within 6-7 weeks. If you really doubt it and think you are, you need an ultrasound done.
If you think & stress about being pregnant more than likely you trick your body into thinking its pregnant, it sounds weird but its also true. Witj all those test being done & not once come out with a positive, i don't think you are. Just relax & take care of yourself, try not to think about it anymore and see if theres a change. If there isn't you should probably visit your Dr. Maybe there's something else wrong.
You are definitely not pregnant. You would be 34 weeks or so if you were (based on the date you had sex) and it would be exceedingly difficult to miss it. Nothing adds up to pregnancy. You have dozens of negative tests, and you're still having periods (number one indicator that you wouldn't be). Your symptoms aren't even just related to pregnancy. Bloating, headaches and back pain are something we as humans deal with on a daily basis. You have a larger belly because you've put on weight - the gain could be from stress, poor eating habits, thyroid problems, etc etc.
It is pretty hard to fool 20 hpts and 5 blood tests. I'd say you're not pregnant.