What Causes Delayed Periods With Severe Mood Swings?

hi im 3 weeks late on my period and over the last few months i am very moody really sensitive and keep getting really emotional one minute i am happy and fine and the next for no reason get bad tempered and moody i am not pregnant my gp did a pregnancy test and it is negative i also did a home pregnancy test and that is negative. my gp has run some blood tests to look at my hormon es but i am worried and want to search for some answers please can you help?