Can an acl injury become more severe a few days after the injury?

Yes: The injury usually happens with a pop and pain, swelling usually follows within 24 hours. Pain may persist or be more severe with meniscus tears or bone bruises.

Not really.: The damage was done with your injury. Several days later you may have more swelling and loss of motion from the injury itself. But once the ACL is torn, it wont become more severe a few days later.