Is Rick dreaming this whole time because if you go back to the first season you see the IV bags but there are two different ones... (at different times) one is half full and the other is full
Is Rick Dreaming?
No. We have a Word of God answer to this question:
On Twitter, Kirkman responded to Uproxx and fellow fans who believed the entirety of The Walking Dead was simply a dream sequence by clarifying that Grimes “is NOT still in a coma” and that the events are actually happening.
Note: For those who aren't aware, Robert Kirkman is the creator of The Walking Dead comic book series and an executive producer on the show. If anyone would know whether or not Rick is dreaming the whole thing, it would be him.
The IV Bags:
A question about the timeline of The Walking Dead was answered by someone who appears to have quite a bit of medical training. Here is his description of the medical equipment we see in Rick's hospital room:
Rick appears to have been receiving a maintenance IV of possibly 0.9 normal saline or Lactated Ringers to keep him hydrated due to fluid loss or possibly a hypertonic solution to decrease intracranial pressure if his coma resulted from respiratory failure. His medical condition would have been stablized prior to the ZA because he is not connected to a vent or any critical drips. The bag's volume appears to be 1000 ml and the rate of administration would be generally 50ml/hr for a man of Rick's size. This bag would take approximately 20 hours to complete before hanging a new bag.
He is not receiving tube feeding. He has no peg tube and no nasogastric tube to receive liquid calories, protein, fats and carbs. There is no feeding pump connected to the IV pole. He has no secondary IV bag or antibiotic hanging with the primary bag in the event of sepsis from his gunshot wound.
Also, Rick's coma probably did not result directly from the GSW [gun shot wound]. Comas unrelated to head trauma can be caused by metabolic abnormalities or they can be medically induced in patients who have suffered respiratory failure and require mechanical ventilation. An induced coma would be done by a Propofol drip through a central IV line. When these drips are stopped it only takes minutes for the patient to awaken.
The scene with Shane standing over Rick with the flowers makes it hard to determine how Rick is ventilated but there is no IV sedative hanging on the pole behind him and later Rick abruptly wakes up with only a nasal cannula in his nose. In a real world scenario Rick needs to wake up very soon to survive. One to two days after the ancillary staff have left the hospital.
If there is a difference between the fluid levels in the IV bag in a single scene, it is almost certainly a simple continuity error. If you're talking about two separate scenes, it would only be natural for the amount of fluid in the IV bag to change over time. When Rick wakes up from his coma, the IV bag is completely empty, which obviously indicates that no one was around when the last refill ran out. As the person I quoted just above said, Rick must have woken up shortly after the hospital was abandoned, because you can't live very long in the summer heat in Georgia with no water.
I actually asked my mother, who is a Registered Nurse, about this issue. She told me that on average, an IV bag lasts for a few hours. In the scene in which Rick wakes up, the bag is clearly empty, and the only indication of how long it has been empty is the fact that Rick hasn't died of dehydration yet. If we assume that an IV bag lasts for 6 hours, then the amount of time needed for a full bag to lose half of its contents would be around 3 hours.
If you are correct in saying that a bag goes from being full to being half full in a single scene, it must be a continuity error. If more time has passed between the full bag and the half empty bag, there is nothing to explain: IV bags are supposed to drain out. If more than a few hours have passed between the full and half empty bags, they might not even be the same bag.