The vaginal bleeding (spotting or menstruation) that you’ve described is definitely NOT a sign of pregnancy. You can take a home pregnancy test to assure yourself. “Implantation bleeding” doesn’t exist. There could be “ovulation bleeding” because during ovulation there is a temporary decrease in estrogen levels. After ovulation, the estrogen levels increase again. Then, the corpus luteum (“yellow body”) starts to produce progesterone that protects the uterus’s mucosa and causesl ovulation bleeding to eventually stop. Soon after ovulation (not more than 24 hours), conception begins in the ovarian tubes. After conception, the fertilized egg travels through the tubes, enters the uterus and embeds itself into the uterus’s mucosa (implantation) 8-10 days after conception. An embedded and fertilized egg cell (whose development began during travel through the ovarian tubes) is now called an embryo. The embryo’s outer layer produces a hormone called "human Chorion-gonadotropin (hCG)" that can be detected by pregnancy tests. This means that pregnancy can be detected 5 days before an expected period. There isn’t any normal vaginal bleeding during implantation as in “ovulation bleeding” (previously described). In the following days, an expected menstruation won’t happen...That is the first sign of possible pregnancy. This is very significant for women with regular cycles; this is not true in your case.
In your case as with many women, irregular cycles have different durations of menstruation. Therefore, you can’t predict them, nor should you connect them with your decision to have sexual intercourse. Oral contraception can help you both to prevent an unwanted pregnancy and to normalize your cycles... but you have to consult a gynecologist first. You can use condom for protection any time you want.