First, I am sorry to hear about the MRI. Back injuries such as yours are serious and are the number one cause of lost productivity in the US economy. You need to address the situation cautiously and intentionally. You should not let your fears drive your decision about what to do although they are valid.
It sounds like you may have suffered a work related injury to your low back that is compensable under the workers' compensation act.. Given your age, some pre-existing degenerative disc disease likely existed before the heavy moving. You will need your medical providers to testify that your condition is work related. Hopefully, you have told the doctors you have seen already that your symptoms arose after this activity. A clear and consistent history, provided to all of your medical providers is very important in a case like this.
You should report your injury to your employer. You have 120 days from the date of the injury to report your claim or you will lose the right to pursue it. If you report the claim within 21 days, you will be entitled to be paid wage loss benefits from the date fo your first absence. If you report between 22 and 120 days, you will be paid from the date of report. Employers and insurance companies rarely agree to pay claims when the report is madelater than 72 hours. Report sooner rather than later.
Pennyslvania law prohibits your employer form terminating you in retaliation for asserting your rights under the Workers' Compensation Act. If your employer has more than 15 employees and you meet the qualifying number of hours, you will have 12 weeks of unpaid leave with job protection under the Family Medical Leave Act. No job protection is provided by the Workers' Compensation Act, although the system has inherent financial incentives motivating your employer to hold your job for a while. If your disability stretches beyond 12 weeks, your employer can lawfully replace you, provided it is not retaliating.
If you avoid telling your employer about this now, because of your fears, you may find yourself in a position, later, where you have to answer questions from your health insurance company. If your medical records indicate the history you have provided, your health insurnace company will want to make sure that workers compensation pays your medical bills.
I recommend that you consult with a workers' compensation attorney. Please feel free to contact me at 610-584-9400 for a referral to a workers compensation attorney in your area who can further explain your rights. Consultations are free and you cannot be charged a fee in a workers' compensation matter unless a workers compensation judge approves the fee. Good luck.