You need to work out how long your luteal phase is hun. Its the time from your ovulation til you get your period. If you count day one of your period as cycle day one..cd1..and you know for sure you ovulated cd 20, and have an average cycle of 28 days based on the average luteal phase of 11-16 days (most having a lp of 12-14 days) that you have a very short luteal phase, which is not good as it wont give the embryo anything to implant in, as the endimitrium wouldnt build up i.e. build up your next period and 'food' for the embryo.
Now, im not saying this to scare you only to suggest that maybe you ov'd earlier than you thought. is that possible?
The other reason im mentioning this is because although your ov can shift due a host of reasons your lp nearly never does, save a day or so. Also, om not sure if you've been timimg your ov for a while, as opposed to just this like cycle, so maybe its possible you ov'd late and as such you'll get your period late....?
Eg. I have a lp of 13 days. On a 29 days cycle i ov on cd 16 and get af 13 days later. On a 30 days cycle i ov on cd 17 etc..
Hope i havent been rambling on..but this info should, toghether with the previous posts about implantation, give you an idea when to start testing. Keep in mind it takes a while for the hcg (pg hormone) to start building and as such its a) difficult to get a bfp early and b) to get symptoms.
Good luck, hun!!!

, Omi xxx