My blood pressure has always been on the low side. For last three days, have been at 145/110 or higher. What could cause a sudden change like that?

Many possibilities. Blood pressure can be affected in the short-term by the level of emotional or physical stress (including pain), excess salt intake in some people, excess caffeine, nsaids such as Ibuprofen or naproxen. If correcting these has no effect, it is important to be seen by a healthcare professional, as there are many other possibilities and too little space to list them.