You do need to be seen again. They should be doing blood tests every 2-3 days to see which way your HCG levels are going.
Once HCG reaches a certain level (1500-1800) they ought to be able to see a pregnancy in your uterus on an internal scan. If they can't then it is likely that your pregnancy is ectopic. Unfortunately many ectopic pregnancies are never seen on scan- this is partly because they are not always in the Fallopian tube.
Good luck, I hope everything is ok and that they take good care of you.
I had the injection for my first ectopic. The injection works by depleting your body of folate so the pregnancy cant develop further and is reabsorbed (i.e. I didn't pass an embryo as with a normal miscarriage).
It is usually effective but does have a failure rate so iirc they monitor afterwards to check your HCG levels are falling. I felt very tired a few days after the jab, possibly this was something to do with the lack of folate, and I took some time off work to recover.
You then have to wait 3 months before ttc again. I was pregnant the month after which was another ectopic in the same tube so I had surgery and then was pregnant with my daughter 3 months after that. It was something of a rollercoaster but worked out in the end.