Three Possitive pregnancy tests, but negative bloodtest

Patient: HiI took 3 pregnancy tests already! 3 weeks apart. First one very faint, waited a week and second test more clear. Same day after second test went for blood test and it came out negative. One week later still feeling nauseus and bloated took third test, which came out possitive again, this time very very clear.Not to know what to make out of it, for i am sterilized aswell…Please help! I am late on my menstrual cycle aswell.

Doctor: