Implantation bleeding is usually very light spotting, so clots point more toward your period or something else going on, not implantation itself.
Beta hCG typically doubles roughly every 48 to 72 hours in early pregnancy, but that rate depends on your starting value, so a slightly slower rise on its own does not automatically mean an unhealthy pregnancy.
Next Steps
Repeat your blood test as your doctor schedules it rather than going by a fixed rule, and get an early scan booked once levels are high enough to confirm the pregnancy's location.
Health Tips
See your gynaecologist sooner if you get heavy bleeding, clots, or pain on one side, since those need checking right away.