Please i don’t understand

So yesterday they did an ultrasound saw the baby and heart beat said am 6 weeks

Now today I went in due to some pain in my lower left side So they did another ultrasound today and there telling me they don’t see anything ? No baby no like nothing and they checked my hcg levels and the only at 4,900 and two weeks ago they were at 3,999 so I don’t get it how did they see a baby and heart beat and a day later nothing !!:/ so upset idk what to think