How many calories do I REALLY need?

I've gotten underweight because of illness/injuries (my BMI is 17.6) and I'm trying to eat enough calories to gain weight back. I can't chew, so I live on protein shakes. I have between 2500 and 4000 calories per day, but I often still feel very hungry afterwards. Back at the beginning of August I hit my lowest weight at 101 lbs and I was also dehydrated and had to go to the hospital and was given fluids by IV, after which I weighed 102.5 lbs. I have now weighed an unfluctuating 102.5 lbs for an entire month, no matter how many calories I have. Maybe I am not getting enough?

The calorie counter on here says I need 2075 calories or something like that per day to gain a pound/week. I'm beginning to think that calorie amount is woefully inadequate.

But 4000 calories per day like my shakes give me sounds like an obscenely high number and I'm scared to have too many calories and just gain back fat instead of muscle. But I definitely need to gain: I have to sit on a pillow because it hurts like crazy to sit on my hipbones, and I can't walk in my bare feet because I hardly have any heel padding left and it's extraordinarily painful to be walking on my heel bones, and I'm tired and weak and I've lost so much muscle that getting up stairs is an ordeal... and if I lean my head back, it falls backwards and almost whiplashes like a baby's, because I have so little neck muscle left...

I'm 5'4", 102.5 lbs, and have always had a very high metabolism. Any idea how many calories I need per day to gain back weight?

If you're not gaining, you're not getting enough energy.  2075 sounds more like a maintenance level.  Most people can't gain on less than 2500 and if 2075 leaves you feeling hungry, you obviously need more.   If you need 4000 a day to get back to a healthy weight and rebuild muscle you need 4000.  Everyone needs body-fat for insulation... it's absolutely not a bad thing to gain body-fat when you have none at all. 

If you're in real pain and struggling so badly physically you should ask your doctor for help immediately.  You may need hospital treatment in the short-term to get you out of the woods.  Longer-term a good dietician would be able to help you devise a healthy diet that meets your nutritional needs and allows for the fact you can't chew.  Protein shakes are useful but probably don't provide you with complete nutrition.  And the weakness you're feeling could be due to nutritional deficiencies as well as not getting enough energy.

Presumably you have a liquidiser?  I'm wondering if you could start devising some smoothies and soups that are very high in cals and vitamin rich.  A good smoothie to try is the flesh of an avocado blended with a combination of olive oil, carrot and apple juice.  A nutritious high-cal soup would contain more oil, red meat/oily fish, lentils, barley, vegetables.  You  should drink plenty of full-fat milk to get calcium and calories.  Also, if you enjoy it, a glass a day of dark stout such as Guinness is very beneficial and improves the appetite.