I'm 20 and unsure if pregnant! I have never had a regular period so the fact that I have gone 3 months without one isn't unusual. The thing is is that I have gained like 5 pounds these last 3 months(last time ihad sex was 3 months ago) I have some acid reflex lately... been real tired and lazy... sometimes I get nauses but to the extent that it is barely noticeable... sometimes when I get aroused my abdomen hurts but that's it. I don't have any other sympotoms. I took a few home pregnancy test all negative. My sister got pregnant a year ago... she was 23 and took tons of home pregnancy test all negative when she was 3 months had blood work it came back negative. And finally after pushing for a scan found she was four months... could I be like her? If so how do I convince a doctor to just do the scan and skip all the other tests

Well you really won't be able to convince a doctor to "skip all the other tests"... for a number of reasons.

But I would definitely suggest setting an appt and dicuss what's going on, and inform them of what happened with your sister.