I think that for reassurance you need to request that the doctor examines your stomach to see if he can feel an enlarged uterus.
If the doctor can feel an enlarged uterus or swelling of any kind, then an ultrasound is required.
If you are not pregnant, then eating extra cheese could be piling on the pounds very fast causing the stretch marks and in which case you need to think about dieting as soon as possible.
Rapid weight gain or loss can affect your menstrual cycle, hence the missing periods. Or an ovarian cyst can also cause missed periods. Or it could just be the implant causing your periods to have stopped.
If you are pregnant then finding out as soon as possible is important for your peace of mind and the care of yourself and the baby.
Either scenario is possible.
With regards to pregnancy tests, as you get later in pregnancy it is possible to get the "hook" effect, where a HPT will show negative because the level of HCG is too high and it effectively overloads it.
The pregnancy tests that you are showing, look flooded, ie you left them in the urine too long.
My sister had a pregnancy where she never had a positive test, she was thought by the doctor to have an enlarged ovarian cyst, sent her for an ultrasound, only to find a 4 month pregnancy. So it is possible that you are pregnant.
However it is equally possible that you might not be pregnant, it sounds from your post that you would like to be, despite having the implant.
You need to see your GP first and get them to examine your stomach first and foremost, if they detect any enlargement/swelling then you can request a scan. Don't go in asking for a blood test or scan initially, as they will not do this just because you ask.