Hello and thanks for the query.
It is thought that the chemicals in celery act to cause sleepiness, increase urine to decrease fluid retention, decrease arthritis symptoms, decrease blood pressure, decrease blood sugar, decrease blood clotting, and muscle relaxation.
 Celery is safe in food amounts and seems to be safe for most people in medicinal amounts.
So the use of celery seeds doesn't cause any joint stiffness rather it helps in decreasing arthritis symptoms.
Side effects of celery seeds
Pregnancy and breast-feeding: Celery oil and celery seeds are LIKELY UNSAFE when used during pregnancy. Large amounts of celery might make the uterus contract and cause a miscarriage.  It’s best to avoid celery in medicinal amounts if you are pregnant or nursing.

Allergies: Celery can cause allergic reactions in people who are sensitive to certain other plants and spices including wild carrot, mugwort, birch, and dandelion. 

Kidney problems: Don’t use celery in medicinal amounts if you have kidney problems. Celery might cause inflammation.

Surgery: Celery can affect the central nervous system. There is some concern that celery, in combination with anesthesia and other medications used during and after surgery might slow down the central nervous system too much. Stop using celery at least 2 weeks before a scheduled surgery.

Blood-thinning medications (anticoagulants) -- Celery seed contains chemicals that may thin the blood. That could make the effects of these medications stronger, raising the risk of bleeding. Blood-thinners include aspirin, warfarin (Coumadin), and clopidogrel (Plavix).
I hope to have answered your query however you may revert to me for any further query.
Best of luck.
Thanks

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Hello
You are definitely advised to use less celery as you are using 75 mg aspirin but a teaspoonful wil not be too much for you and hence you can take it.
Thanks

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