Here's a strong hint. Food = repairs muscles, aids soreness. Make sure your PWO shake had enough macros and you eat properly post post workout.

Lol. Most of you guys are overzealous in your diagnosis. First, pain is relative. There's a difference between real injury and DOMS (Delayed Onset Muscle Soreness) due to lactic acid build up.
It depends on a multitude of factors guys, whether your nutrition is sufficient enough to boost recovery, especially post workout and post post workout.

Cos your body has adapted to it. I used to have reservations abotu squatting 3 tiems a week when i started the madcow program but it's not that bad. My squats are actually getting better, but plateauing at 310lbs cos of some lower back issues. I'm gonna stop deadlifting fo a few weeks.

You know, sometimes i wonder about that. There's a school of thought saying if your muscle adapts too much, there's no more growth stimulation and periodization or a new stimulus is required.

Then another group says new stimuli is in the form of poundage progression. Bigger weights = more hypertrophy.

Whey, honey, even oats & maltodextrin mixed with water for postworkout. You need lots of carbs. Stay away from fats, even healthy fats like fish oil, etc because they slow down absorption of protein.
The fat in milk is still hotly debated, some argue that the sugars in milk get the anabolic cascade running faster but others disagree due to milk fats. Up to you here.

Whey is a by product which is derived from milk, which has 2 components; whey and casein. You can order whey through maxtrition.com.my which have good prices. (sorry for the short advertizing m'potato). I recommend Optimum Nutrition's 100% whey. It's at the "protein" category.
Maltodextrin is a form of high GI carbs but you can use good old fashioned instant quaker oats or 2 tablespoons of honey.

If it's more than a week, could be a muscle pull or injury. Get it checked or take some time off. Don't screw with this, cos the abs are your core, you need it in every exercise regardless of what bodypart you do.
Time off/getting it checked > not being able to lift for good.