Hi Doulas thanks for your question.Your having swelling left side of pubic area above penis,the area is called as inguinal region and your also feeling tightness and swelling in this area an swelling is more on standing.
From the description of the swelling it looks to be a classical case of inguinal hernia
,but since you have been examined by GP and urologist
and they ruled out hernia.Hernia is a reducible condition,there can be possibility that when you went o Doctor at that time your hernia had got reduced and they could discover none.
Since your doing lot of exercise there could be possibility of strained muscle or a small haemotoma (collection of blood under skin,due to trauma)Get your self re examined by Doctor when swelling is more prominent.Hope i answered your question