Hello.,
Blood clots in semen is known as hemospermia / Heamtospermia.
Causes can be abstinence from sexual activity, vascular malformation, infection or inflammation of sexual glands like prostate or seminal vesicle. This is not really a serious or risky conditions but if it keeps happening an evaluations is warranted. Most the cases the causes will minor like a urinary tract infection.
The investigations required are :
Semen Analysis.
MRI PELVIS. (to be considered at a later stage)
Urine routine examination.
Per rectal examiantion
STD tests
Urine culture
These tests are mainly to make sure 'every thing is alright'. You need to get in touch with your GP / Urologist for this.
Hope this helps
Feel free to ask followups / calrifications
Regards
Dr Gautam Banga