Ovulation or Implantation?

Hello,
My cycle is irregular every month; sometimes it comes on Day 25, 26, 27. Rarely on Day 28. Last month it came on Day 25, on February 18th. Lasted until the 23rd (for 6 days).

On Tuesday, March 8th, I discovered a very small dark red spot in my underwear. That was on Day 19 of my cycle. The following day, March 9th, on Day 20 of my cycle, I noticed at the end of the night a much bigger dark red/brown spot, with several smaller spots surrounding it. There have been no more sightings of the spotting; however, I have had small twinges in my lower stomach region on and off the past day and a half. I also woke up the following day after the much heavier spotting with intense lower left back pain, that receded and eventually faded all together with no recurrence after about 2 hours. I have been excessively tired and moody, and slightly bloated, with a headache this morning. My breasts SEEM a little larger, but that could be due to my overly-obsessive researching of the signs and symptoms of implantation bleeding.

I read that during ovulation, mittelschmerz occurs and blood can be the result. Could the spotting be just old blood from ovulation? I did have sex with my boyfriend of 4 years around the time of ovulation, unprotected (we live together)..Or could it be implantation bleeding? What is the difference between the two?

Today is Day 23 of my cycle. I am very nervous, and hopefully an answer can be provided to me soon

Thank you for your time!