Pregnancy test at the doctors. 😵😵😵

So yesterday I went to the doctors to get an xray and of course they need to know if your pregnant or not. I haven't gotten my period yet I'm 20 days late and didnt know if I was or not. ( I have been trying for 2 years now to get pregnant so I was waiting 1 more week before taking one.) Anyways ... so they did a pregnancy test and they said it went positive and then the line dissapeared ( it did that 3 times) so they couldn't tell me if I was pregnant or not. They told me to take one at home. But I'm to scared to I dont want a negative result. Has this ever happened to any one??? Can't stop thinking about it. Want to know what my chances are. 🤯🤯🤯