If I understand you correctly - the first line is Jan, Feb, Mar etc. In A1 type Jan and then put the cursor on the bottom right corner of A1 so it changes to a solid cross, hold down the right mouse button and then drag across to L1. This will give you all the months (it will work whatever month you type into A1). You can then enter the current weight in A2, A3 etc.
In B3 type =A2-B2 and then drag across as for the months. This will give you the change from the previous month.
In B4 type =($A$2-B2) and copy across as above. This will give you the overall change.