Kass... So much to know hunni...

CD1 is classed as the first day of bleeding... if you typically spot for a day then I would count that as Day 1, otherwise its the first day of bleeding.

How do you know when to dtd... how long is your cycle usually? Mine is typically 28 days - which is somewhat unusual really. So I dtd either every day or every second day from about CD10 to CD17 to make sure cover any potential surprises in cycle changes.
Best way to know you are pg is if you are late.. HPT's can often not pick up the hcg for some women or can show early results so not entirely reliable. If you bleed at regular AF but its really different to usual, its a possibility.

Best way I found to understand my body was to chart my cycles. I got a BBT (body Basal thermometer) which shows 2 decimal points for better accuracy. And using the program Fertility Friend (FF) online you can track all of your information to see what your body's signs are of ovulation and pms etc.

Oh and there's cervical mucous changes as well... but FF will also explain that to you in detail.

HTH.