Yellowing sounds like a mineral deficiency and can be caused by many things.
Overwatering can cause leeching in which the nutrients leech from the compost leaving the yellowing of the leaves.
Yes, it can be caused by under watering but you would normally expect other symptons here such as browning of leaves. Dropping and wilting. And since you mention you only give it a little water each week, that does not sound like the issue.
It may even be the wrong light conditions indoors - depending on the type of plant.
So there is no quick answer to this. Firstly, check the type of plant you have and have a look online for the correct maintenance regime for that plant. Depending on where you live, you will find loads of reputable groups specialising in most plant genres. In the UK the RHS are a good place to start. As is Gardenner's World guides.
If the plant is old and has never been repotted, then it may need repotting. That may also mean potting up a size if it is becoming 'pot bound' - when the roots are dominating the compost and need more room to develop. It may simply be that the compost is now 'spent' and dried out. No longer retaining moisture and without any nutrients.
I am not sure if you ever feed your plant, but they do need feeding - even indoor plants. Well, most do.
So, there are a number of things to do. But start with checking the maintenance regime for your plant oonline. Then consider repotting if old.
