All sorts of things can affect the signal strength your aerial receives. As you state, it's possible that your neighbours might have moved something inside their house. Or either you, or they, might have plugged in a device which is causing electrical interference to an already weak signal. (All electrical devices emit electromagnetic radiation. Simply plugging in a new printer, or even a fridge, could cause the problem).
Additionally, nature can play some strange tricks. Sometimes a tree coming into leaf can be sufficient to interrupt the path between the transmitter and your TV. The time of year can also make a difference. (The height of the reflective layer of the atmosphere, which bounces radio waves, changes between day and night. As days get longer, the reception of radio signals can change. Even if the signal you require isn't directly affected, other signals which interfere with and weaken your signal might be altered).
Lastly, sunspots on the surface of the sun have a significant effect on radio communications on earth. They come and go on an 11-year cycle and are often responsible for 'unexplained' reception problems.
All of that simply means that you're right. You do need to replace your aerial. However, you're obviously in quite a good reception area because most people can't get any digital signal from a portable aerial. There's quite a good chance that you could use the same aerial as your other digi-box uses. You simply need to put a splitter box in the cable and run a new cable to your kitchen.
Chris