Since your periods are irregular .ovulation might have occurred little late..so that's why tiny sac at this stage .what dr has told is correct ..
U have to wait atleast 2 weeks for pregnancy to appear on scan properly.
For cramps you may have to take additional progesterone support .
Next Steps
you can visit obstetrician and take progesterone medications and repeat scan after 2 weeks
Health Tips
Do not lift heavy weights .don't take any stress .