Spotting

Hi moms, patulong po. Medyo kinakabahan at paranoid ako lately. Just gave birth 5months ago, normal delivery and formula-fed si lo. Nag-start ako magka-period, June 4. Then last menstruation ko is Sept. 5-11. Nag-do kami ni hubby, Sept. 15 & Sept 18. always withdrawal, 4months after ko manganak kmi nakapag-do. Then nagulat ako, may brownish discharge ako Sept. 20 & 21. Konti lang naman yung discharge. Bakit po kaya ganun? Possible din ba ako mabuntis agad?