How would you know if it is belly or pregnant?

Urine pregnancy test. A "baby bump" belly is a feature at 4 or more months, so it is certainly far along enough that a simple urine pregnancy test would be positive.

Only by testing specifically in early stages. Get your pediatrician involved for a visit and discuss your worries as otherwise anxiety will build up.

If you had sex since your last period and missed your period by 4-5 days, do a home pregnancy test. If the result is negative repeat the test in 4-5 days. Use first morning urine and follow instructions for the test carefully. If you do not wish to be pregnant, use contraception all the time, every time. You may consider implanted contraceptive, or IUD. Get HPV vaccine.