When you are short of cash buying gifts for special occasions is a problem. The answer is to do something that demonstrates that you appreciate your mother without spending money you can ill afford.
If you can come up with an ideas for something she will like, then it does not have to be an expensive gift nor does it have to be a big or costly gesture. Most mothers like to get gifts that are original and made especially for them.
All it really needs is to create or find something that shows that you thought about her, remembered her and that you care.
Most mothers will know if their kids are short of cash, in which case they would rather receive a card with a kind and loving message than see you struggle to buy a gift you cannot afford.
With a bit of thought and a little investment of time you might surprise yourself with what you can produce. Here is a page with ideas for low cost gifts and cards (some are free) to show your mother that you care about her.
If you take the time to make something specially for your Mom it shows you care.