Doctors Say I Might Miscarry! I'm Scared Help?

I went to the hospital on nov. 1st b/c I was having cramping pains in my lower stomach than it would get higher and swicth places from low to high from left to right and they told me I was pregnant which I knew b/c I missed my period on the 29th of september and I still have these side pains no bleeding though so they did some blood work which showed I was 4wks going on 5 that next day pregnant now i'm 6 going on seven tomorrow they did a ultra sound which was very painful from the cat. To the pressure from the water! But they told me they seen a sack but no baby which could mean to things I was very early fore them to see anything or I was about to misscarry I got very scared he told me to rest and if I were to see any bleeding come back when they took the cat. Out I bleed it scared me but they say it was normal to bleed from it but all these side pains got me thinking tubal preg. Or something bad this is my first preg. I know nothing but my sides kill me can someone help me and let me know what they think! Why couldn't they see my baby?????