Could "straining" or putting too much pressure on your neck cause a brain bleed or aneurysm? or would you have to have already has an aneurysm forming?

A muscular neck strain would not result in a brain bleed or cause an aneurysm to form. It would cause neck discomfort, especially with moving, and rest will improve it along with acetaminophen and/or ibuprofen as well as either heat or cold. If the strain doesn’t improve with these measures, then see your doctor. Good luck!

At your age, if an aneurysm were present, it would most likely represent a congenital berry aneurysm. Post-traumatic aneurysms are not caused by straining and pressing on your neck is not a cause of brain hemorrhage.