A couple of things...the tub may not be sitting on the floor, or is not fully supported by the flanges, or the floor is not strong enough to hold the weight of the tub, water, and person without noticeable deflection. If the long sides are properly supported, the short sides probably won't add much. You could try cleaning the caulk out of the seam, fill the tub with water to cause it to open up the gap, then recaulk it. Leave the water in until it sets (at least overnight - some caulks don't actually fully cure for a week or more). My unprofessional opinion.