Welcome to the forum huiyan, I know its probably not the best time for welcomes, but welcome all the same
Im sorry to hear what happened to your hamster. I agree with you that you should have been more responsible, however I wont condemn you as being a bad hamster owner. Sometimes things just happen that we didnt intend for, it doesnt make us a bad person. However its important that you learn from the experience as I am sure you will
You are showing a level of responsibility by looking for help in places you know you may be able to get it. You have already been told how serious it can be for a hamster to show any amount of blood. I understand you cant afford a vet, But here is the bottom line from my perspective.
Number one: Just let the hamster be for a time, its good to know that it is acting normal and eating and so on. Those to me are good signs, However hamster are able to hide any illness or injuries pretty well at first, but if something is internally wrong they will start to deteriorate fast once they show the symptoms of illness.
At this point I would just say to keep and eye on the hamster, provide it a quiet place and dont handle it for a day or two, (meaning dont take it out for playtime) but make sure you keep a very close eye on its health and attitude.
Number two: There is always a chance something may go wrong, especially with something as bad as a big fall. If you cant afford a vet then I suggest you start looking for people who will lend you the money and are able to in the event you do need to rush your hamster into a vet. You can always find a way to pay them back. If not then perhaps barrow money from your parents.
Another option is the bring your hamster to a shelter and put it up for adoption they will treat your hamsters injuries and can find it a home to live with a person who is able to finically support any vet bills.
Im sure you understand at this point that nobody should ever take on the responsibility of an animal unless they know they can afford it or are willing to barrow money from other places for vet bills incase you happened to be low on cash at that point. So you may want to consider giving your hamster up for adoption if you know you cant afford a vet, because if the hamster is fine now you still never know if you may need one again in the future.
Safe Play Areas
There are special rodent playpens you can buy that are out there. Or you can just creative and make a play area on the floor out of boxes or whatever you might have that would make a good solid barrier. When you play with your hamster put it in this confined enclosure with toys and such and make sure it has constant supervision. If you plan on handling your hamster elsewhere then so in a save area. Try sitting on the floor or perhaps sofa, anywhere that the hamster cant fall and injure itself.
Good luck though, and please keep us updated.