Spotting right after intercourse, especially just after your period ends, is often from minor irritation of the cervix or the lining still settling down, and red brown discharge is usually just older blood working its way out. The back pain and lower belly ache you're describing often go along with this, but the weakness is worth not ignoring.
Next Steps
Rest, stay hydrated, and track how many days the discharge and pain continue. If it hasn't settled in 2 to 3 days, or the bleeding becomes heavier than light spotting, see a gynaecologist for an examination.
Health Tips
Go in sooner if you feel dizzy, very weak, or the pain turns sharp and one sided.