Based on the information provide it seems that very minimum/few chances to be pregnant, here is the reason to back it up.
1) you just have a baby 5 months ago , and most probably you are breast feeding the child, if so the breast feeding help shield against pregnancy
due to various hormonal
changes occurs in the body due to breast feeding (prolactin-hormone - decrease the release of estrogen and progesterone
and hence are having irregular periods
2) you took IPILL - with in 72 hours so its second point for relief.
3) urine pregnancy test is negative implies the conformation that you are " not pregnant "
1) REST ASSURED THAT YOU are NOT PREGNANT.
2) please stop using IPILL as protection against pregnancy (way harmful in repeated uses).
3) try using condoms or other means of protection except ( no oral contraceptive pills
4) the irregular menses is due to breast feeding it will come to normal cycle gradually one you will stop breast feeding.
Hope it helps.