The knot under your arm is most likely not related to your thirst and inability to sleep well. However I think that you may be anxious about what is causing that knot and therefore you are not sleeping well and your mouth is dry. You need to have the knot or lump examined by the doctor to know exactly what it is. Extreme thirst could be a symptom of diabetes. You can also discuss this with the doctor and he may run a fasting blood glucose on you to check for diabetes. Salty foods will make you thirsty so avoid them for now. I recommend relaxing before bed to encourage better sleeping. Take a hot bath, read a book, do some deep breathing in and out to relax you. You could ask the doctor for a mild medication to help you sleep, as well.