I have very irregular periods since going on the injection and had one until approx. 1 week ago. I am now having the following symptoms:
Do you mean you were bleeding until a week ago? If so, it's highly unlikely (although not at all impossible) that you'd get pregnant from intercourse approx. 14 days ago. Most women ovulate "about" 2 weeks before the first day of their next period, so figuring out when you ovulate can be tricky if you're irregular. Some women ovulate irregularly, though, which can make it even harder to determine.
However, I should ask: was it a period or a "period"? The first symptom of all my [four] pregnancies was implantation bleeding that for all the world resembled a short period. This is not the norm, though.
When was your last injection?
The symptoms you described are common for ovulation as well as pregnancy. Tender breasts/nipples would not come until later [usually]. I'd ballpark it at 4 to 6 gestational weeks, but that's just a estimate. Increased urination is due to the pressure of the fetus on the bladder, so that wouldn't come until much later.
Thick, white discharge can
be a symptom of pregnancy, but is almost always a symptom of ovulation.
The pain in your abdomen is worth mentioning to your OBGYN, but is probably unrelated. Pain from implantation is fairly rare from the little I know of it. I get those from time to time, and it's usually just a muscular pain. When you think you may be expecting, every little twinge can turn into a pregnancy symptom.