Hello, I believe you have sprained your thumb while playing lacrosse that resulted in black and blue under the nail. This is known as Game Keepers thumb and is commonly seen in football goalkeepers, skiers and rugby players. If there is sprain it results in thumb swelling and bruising which may develop in a few days. The ability to pincer grip small objects between thumb and the index finger after severely impaired occurs due of the resultant instability in the metacarpo phalangeal joint. There will be pain and tenderness over the ulnar collateral ligament of the thumb. Diagnosis is made by physical examination. Stressing the ulnar collateral ligament reveals instability in the joint. This can be confirmed by taking an X ray. Ice packs and compressions are the best treatment .Non steroidal anti-inflammatory drugs like tablet ibuprofen twice a day after food for a week can be used to relieve the pain. They usually resolve by 4-6 weeks and can be aided by physiotherapy. However it should be assessed first by your orthopedician who can do a complete evaluation. Whether you need a surgical intervention, depends on the extent of injury. Even preventive taping can be effective in improving thumb joint stability and preventing ulnar collateral ligament injuries. If there is a stenar lesion, then surgery is essential. I would suggest you an X ray of the hand involved and review the reports with orthopedician. Hope this helps. Thank you