I've had pain and aching in lower abdomen for 48 hours. The area feels firm and tender when pressed. Any ideas what might be the cause?

Time for exam. It could be a urinary infection, inflammation of your bowel, even appendicitis. It might be a muscle strain or a hernia. In any case it is very worth your time and the inconvenience of having a medical practitioner examine you and maybe order some tests. Don't worry about it, just do it.