There are many sensations that happen with qi gong and tai chi, perhaps particularly so in the beginning of practice. Shaking, burning sensations, strong movements, cool spots, etc. are all somewhat common. Generally they pass within months of training and you should just try to ignore the sensations if you have no underlying symptoms. Some teachers will recommend techniques like breathing into the area, covering it with light, etc. - but I always recommend to just let it go and let the body do what it needs to do to open.
If you are overweight and you are feeling pain in the lower back it may be from your stance, so you should have that checked to make sure you are allowing your weight to go to the ground and not get stuck in your hips (assuming you are doing some kind of standing qi gong).
With regards to the spleen deficiency, you may have this, as it is common with weight issues (as it leads to dampness). This, however, has nothing to do necessarily with the physical functioning of your spleen. So as you learn more of qi gong and Chinese Medicine it is good to not too closely correlate the meridian systems with the corresponding organ - particularly with regards to the possibility of physical malfunctions. The diagnoses are speaking of imbalances in meridans with regards to other meridians in the body - not physical problems. It's good you have been checked out by a doctor, but now that you are fairly sure nothing physical is wrong those diagnostic ideas are best used within treatment planning and not within your mind.
For spleen qi deficiency you want to avoid dairy, fried foods, cold/raw foods, etc. and focus on cooked foods that are easy to digest, at least some meat, particularly red for yang energy, and light exercise for which qi gong is perfect.