When EXACTLY did your last period start? It matters. Was it after the 24th (start of last week). I'll base this upon that date:
Normally you'll ovulate between day 11 and 16 of your cycle. Sperm can survive about 5 days. The egg only is viable about 48 hours after ovulation.
You had sex on day 3 or 4 with your coworker. It was likely early for you to conceive.
January 11 would have been day 18. Late but possible.
See why we need that date? Still, it looks like the boyfriend is the father. Only a DNA test can confirm.