positive pregnancy test.. but low beta

Sw

Hello... My period is 10days late.. Took a hpt.. It was faint positive.. Went to doc office.. They took pregnancy test it was strong positive... They took serum beta.. It was 36 only..doc told me to repeat the test after 2 days.. I am really worried.. Anyone had these low beta.. Top one take couple of days back at home..bottom one took by doc office