Why do a breast ultrasound after a mammogram? Does it mean there’s something abnormal in my breast, or was the mammogram just not clear? My doctor won’t talk to me until after the ultrasound, but i’m worried now.

It means that. there was an abnormality or potential abnormality on the mammogram that required additional evaluation or confirmation with ultrasound. Or, if you have dense breasts, screening ultrasound may be performed as an adjunct test even if the mammo shows no abnormality.
Complete the workup. What is true: mammogram was not completely normal. You need additional testing to try to determine the cause and significance of the finding. If a new finding, first step would be spot mammograms and an ultrasound of the area. Usually the area of concern is due to overlapping structures that can be resolved with ultrasound or extra mammo views.
Addtional Testing. Ultrasound is an additional test to evaluate a finding on a mammogram. If there is a lump, ultrasound can determine cystic vs. Solid (mammogram typically can't make that distinction). If the mammogram shows a density, ultrasound may be able to determine what caused the density. It is not a better test, and does not mean that there is a 'bad' problem.
Your . Your healthcare provider most likely wants to examine some tissue further. Ultrasounds can detect tissue abnormalities using ultrasound. It is prudent to think that further tests are being done because your mammogram showed abnormality that required further testing. Trust that your doctor is taking necessary steps in favor of your health.