I usually walk 5 km everyday and suddenly thought of doing squats without any warm up. I did 20 repetitions, 2 times, and never felt any pain at all. In the evening, I felt stiffness, but then, I was not able to sleep or sit because of that. It's been 2 days, and I am still feeling the same. What could be wrong? Are my thighs injured? Should the cramps last more than days? How should I handle this? I am not able to sleep, I feel pain below my buttocks, and I am not able to use steps. I am feeling really worried. Should i go to a physician?
What you are feeling is normal. Any time you do a new exercise, or even an old exercise in a new way, you will be pushing your muscles past the point where they are comfortable and making them sore. For the most part, this is lumped into the term Delayed Onset Muscle Soreness (DOMS).
For some people, the presence of DOMS becomes a goal or a indication of a good workout because, as you do the same exercise over and over again, your muscles get use to the activity and no longer need to adapt. As a result, serious lifters will try to change up their workout to re-engage their muscles - usually by increasing the weight.
For you specifically, you are sore because a) you are doing an exercise in a new way and b) you are engaging the largest muscle system in your body - your quads and glutes. The good news is that this will pass. Further, you will probably never feel this sore again.
If you still hurt, take a hot bath, give yourself massage, and then go for a walk. The soreness will subside on its own in a couple days.
I primarily agree with Sparafusile. DOMS is pretty normal, especially for those impulse exercise sessions where you haven't warmed up, or haven't done the exercise before. Over the counter painkillers and anti-inflammatories such as aspirin or ibuprofen combined with heating pads or hot baths will give you enough relief to be able to sleep. For steps, as silly as it might sound, if you're having trouble lifting your legs, try going up them backwards or sideways. It uses different muscles, so sometimes it can work as a short term workaround.
Lastly, if the pain is extreme or is starting to interfere with your ability to get around or to work, consider seeing a doctor. It is possible to injure yourself in a way that doesn't become evident right away, there are some serious conditions such as thrombosis that can be caused by exercise (although they're rarely seen in more casual sessions such as you did), and a $50 doctor's visit that nets you a prescription for a stronger painkiller might mean the difference between missing work and being able to continue making money.