Help.. Anyone please :/ scared and don't know what to do

I'm scared and confused on making the right choice. If anyone knows or suggestions. I'd be greatly appreciated. I had a positive pregnancy test, 2, last Sunday. I had really bad cramping pain Tuesday, had to go to the ER. They did blood test and ultrasound. My HCG level was only at 354 and couldn't see anything in ultrasound, but said might be too early to see. I go back 48hrs to get my blood drawn again, my HCG leveled dropped to 258.. Now Monday I went to the OBGYn, they did all the same, confirmed pregnancy test and all. But they still don't see anything in uterous or even fallopian tube they said. I won't get my blood results till today when the doctor calls so not sure what I'm HCG level is. But they said I still need to come back Wednesday for blood work. If HCG still isn't double or raising. I should come back Thursday for the injection shot Methotrexate to stop all pregnancy... Reading about the shot if scary ... Any comments would be appreciated. Being my first pregnancy, I'm alittle scared on what the best option is.. Do you think the baby is just small or too early to see?. I don't want to have an abnormal baby if the shot goes wrong... Help