Why clothes get tighter:
1) they shrink
2) you gained muscle
3) you gained fat
4) you gained a combination of muscle and fat
5) you are retaining water
6) you are bloated (maybe due to food, constipation, or PMS)
7) it is in your head
The only way you can tell if you are gaining muscle, fat or both is to get your body fat tested regularly. Ideally, you will have a Bod Pod or Hydrostatic near you. If not, find a trainer you TRUST to take your body fat with calipers.
When you get this number (a percent), weigh yourself. With both your body weight and your fat percent, you can find the number of lean pounds and the number of fat pounds. Also, if you get a caliper test, write down the measurement of each spot tested.
NO LESS THAN 4 WEEKS LATER, have the SAME person do the test again. Since you have the measurement of each spot, you can compare each spot, as well as total numbers to measure progress (or lack thereof).
Also, as Figureskater said, a good way is to take a tape measure and measure a few spots. Then, 2-4 weeks later, measure again. Good spots are bicep, chest, waist, hips, thighs, calves. You can record these numbers and measure progress (or lack thereof).
Photos are also very good.