"You can't get pregnant right after your period" is one of those myths that just refuses to die. For a lot of people the days right after a period really are lower risk. Lower risk is not the same as no risk, though, and for some people those days land squarely in the fertile window.
How the fertile window works
You can conceive from sex during a roughly six-day fertile window: the five days before ovulation plus ovulation day itself. The reason is timing. Sperm can hang around in the body for up to about five days waiting for an egg, while the egg only lasts 12 to 24 hours once it is released. The ovulation calculator gives you a rough window to work with.
Why "right after my period" is not always safe
Picture a short cycle, say 21 to 24 days. You might ovulate as early as day 8 or 9. Sperm from sex on day 5 or 6, just as your period wraps up, can easily survive long enough to meet that early egg. Long or irregular periods narrow the gap even more. So whether "right after your period" is genuinely low risk comes down entirely to your own cycle length and how regular it is.
Why the calendar makes lousy birth control
Ovulation drifts from one cycle to the next with stress, illness, or travel, so no calendar method holds up as contraception. The cycle risk calculator is handy for seeing roughly higher and lower-risk days when you are planning, but if your goal is to avoid pregnancy, reach for a proven method instead. Knowing your usual cycle from the period predictor at least helps you read the timing.
So, where does that leave you?
Conceiving right after a period is less likely than mid-cycle, but it is far from impossible, especially on a short or unpredictable cycle. Use calendar timing to plan, never to protect.