tiger the way I see it your cycles are usually 30 days long (you count from the first day of bleeding to the last day before you start bleeding again. so that's 9 days of bleeding plus 21 days between = 30 days).
The luteal phase is basically the time between ovulation until when the new period (=new cycle) begins. For each woman, this phase is normally always the same length. It can be anywhere between 12 and 16 days long (if it is shorter, which also occurs, it will be increasingly difficult to conceive). So if for you it is 13 days, it is normally always 13 days for you. And if it's 15 days, it should always be 15 days.
So with a 30-day regular cycle your ovulation would be expected to be sometime between cycle day 14 and cycle day 18 (counting from the first day of your last period!).
I reckon from your description that you are now at cycle day 18 or 19. So if your luteal phase is very long and you ovulate on CD14, you would now be 4-5 days past ovulation (DPO). Sometimes but very rarely implantation occurs this soon. And sometimes you do bleed a little with implantation.
So what you are experiencing might be implantation bleeding, but it isn't very likely.
On the other hand, if your luteal phase is short and you ovulate on CD18 (or even later), then you would just about be at ovulation time now - so any bleeding could be associated with ovulation.
Was your last cycle regular and your period bang on time? How is the bleeding progressing?
About fertility signs, I recommend you sign up with www.fertilityfriend.com. When you sign up you get a free online course that explains everything. Some people who have regular cycles always miss their most fertile days because they just assume that they ovulate on CD14, when in fact they may ovulate much later or indeed earlier. Charting your basal temperatures and observing your cervical mucus can give you very useful indications for timing your intercourse succesfully!