U could be well prego.
I am also an irregular period goer who is ttc as well. Spotting occruring afew days after sex could mean implantation bleeding but not 100% confirmed. U can do a home prego test and find out in a week time. U will know whether u r or not then.
Anyway ur period is due on 26/9/2005??? See if it comes along then.
Some common early prego symptoms, to look out for will be,
1) nipples sore (when touched, painful)
2) aerola darkened (the dark part around the nipple)
3) funny feeling at the stomach
4) bloated feeling
5) nausea
6) & others...