I am unclear as to your surgical description of the procedure you underwent. I understand a discectomy but do not understand a “lisectomy”. You apparently underwent a fusion at L5-S1 but normally, four screws and rods are used. By your description of “L5-S1 plates on both sides and 6 screws” leads me to believe that you had L4-S1 fused as six screws are used with a two level fusion.
Scar tissue and “a huge amount of liquid in the area of surgery” indicates a seroma, hematoma, infection or dural leak. This collection of fluid can cause nerve compression leading to buttocks and leg pain.
Two liters (2000cc) of blood loss from a fusion surgery is a very large volume. I normally lose from 50-200cc with that type of procedure. The extreme blood loss is confusing. Drains are normally left in for 1-2 days. Removing after one week is something that is distinctly abnormal and with continuing drainage even then is concerning. Could this be CSF from a dural leak?
Headaches that are positional (become worse with being upright and get substantially better when you lie down) can be dural leak related.
How long ago was surgery performed and why was the original reason for the surgery? What did the original surgeon state regarding the performance of the surgery? Are you having chills, sweats, fevers and a temperature? Were there any lab tests performed and if so what were the results?