First of all, cut out the Immodium. Long term use is not good.
Take Metamucil twice daily. This is water soluble and is safe to take. It will bulk up your stools and stop diarrhea. Drink plenty of water with it though so your belly does not get bloated. It may do at first, but your system will get used to it.
Also, start to eat live yogurt or take Acidophilus caps twice daily too. This will rebalance the good bacteria in your gut that you lose a lot of during diarrhea. Lack of this also creates bloating and gas. Keep them in the fridge and take them twice daily on an empty stomach.
If you have constant diarrhea, you may like to take a Calcium supplement. This helps the bowel to absorb more water naturally and will naturally make your stools bulkier.
When you move around and are standing up, you are doing more exercise and this will increase the peristaltic action of the gut, hence why you need to have more bowel movements. If you are sitting down all day, this slows peristalsis down and the bowel becomes more sluggish.
Take a look at your diet also and cut out junk food, soda and coffee. This can make diarrhea worse. Eat carbohydrates such as potatoes and also boiled rice is good and this absorbs within the stomach and will lessen diarrhea also.